diff --git a/app/i/[id]/__tests__/InvoiceDetailPage.test.tsx b/app/i/[id]/__tests__/InvoiceDetailPage.test.tsx
new file mode 100644
index 0000000..04b926b
--- /dev/null
+++ b/app/i/[id]/__tests__/InvoiceDetailPage.test.tsx
@@ -0,0 +1,114 @@
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import InvoiceDetailPage from "../page";
+import { getInvoice, type Invoice } from "@/utils/soroban";
+
+vi.mock("@/components/ActivityFeed", () => ({
+  default: () => <div>Activity feed</div>,
+}));
+
+vi.mock("@/components/ChangeInvoiceTokenModal", () => ({
+  default: ({ onSuccess }: { onSuccess: (token: string) => void }) => (
+    <div role="dialog" aria-label="Change Token">
+      <button onClick={() => onSuccess("CEURC")}>Mock change token success</button>
+    </div>
+  ),
+}));
+
+const connect = vi.fn();
+let walletState = {
+  address: "",
+  connect,
+};
+
+vi.mock("@/context/WalletContext", () => ({
+  useWallet: () => walletState,
+}));
+
+vi.mock("@/hooks/useApprovedTokens", () => ({
+  useApprovedTokens: () => ({
+    tokens: [
+      { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" },
+      { contractId: "CEURC", name: "Euro Coin", symbol: "EURC", decimals: 7, iconLabel: "EU" },
+    ],
+    tokenMap: new Map([
+      ["CUSDC", { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" }],
+      ["CEURC", { contractId: "CEURC", name: "Euro Coin", symbol: "EURC", decimals: 7, iconLabel: "EU" }],
+    ]),
+    defaultToken: { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" },
+  }),
+}));
+
+vi.mock("@/utils/soroban", async () => {
+  const actual = await vi.importActual<typeof import("@/utils/soroban")>("@/utils/soroban");
+  return {
+    ...actual,
+    getInvoice: vi.fn(),
+  };
+});
+
+const invoice: Invoice = {
+  id: 9n,
+  status: "Pending",
+  freelancer: "GFREELANCER",
+  payer: "GPAYER",
+  amount: 10_000_000n,
+  due_date: 1_900_000_000n,
+  discount_rate: 300,
+  token: "CUSDC",
+};
+
+function params(id = "9") {
+  const value = Promise.resolve({ id }) as Promise<{ id: string }> & { _resolvedValue?: { id: string } };
+  value._resolvedValue = { id };
+  return value;
+}
+
+describe("InvoiceDetailPage", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    walletState = {
+      address: "GFREELANCER",
+      connect,
+    };
+    vi.mocked(getInvoice).mockResolvedValue(invoice);
+  });
+
+  it("shows Change Token only for the submitting freelancer while pending", async () => {
+    render(<InvoiceDetailPage params={params()} />);
+
+    expect(await screen.findByRole("heading", { name: "Invoice #9" })).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Change Token" })).toBeInTheDocument();
+  });
+
+  it("does not show Change Token for non-submitters", async () => {
+    walletState = {
+      address: "GOTHER",
+      connect,
+    };
+
+    render(<InvoiceDetailPage params={params()} />);
+
+    expect(await screen.findByRole("heading", { name: "Invoice #9" })).toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Change Token" })).not.toBeInTheDocument();
+    expect(screen.getByText("Only the submitting freelancer can change the invoice token before funding.")).toBeInTheDocument();
+  });
+
+  it("does not show Change Token after the invoice leaves Pending", async () => {
+    vi.mocked(getInvoice).mockResolvedValue({ ...invoice, status: "Funded" });
+
+    render(<InvoiceDetailPage params={params()} />);
+
+    expect(await screen.findByRole("heading", { name: "Invoice #9" })).toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Change Token" })).not.toBeInTheDocument();
+  });
+
+  it("updates the visible token immediately after a successful modal change", async () => {
+    render(<InvoiceDetailPage params={params()} />);
+
+    fireEvent.click(await screen.findByRole("button", { name: "Change Token" }));
+    fireEvent.click(screen.getByText("Mock change token success"));
+
+    await waitFor(() => expect(screen.getByText("EURC")).toBeInTheDocument());
+  });
+});
diff --git a/src/components/__tests__/ChangeInvoiceTokenModal.test.tsx b/src/components/__tests__/ChangeInvoiceTokenModal.test.tsx
new file mode 100644
index 0000000..d66c3a3
--- /dev/null
+++ b/src/components/__tests__/ChangeInvoiceTokenModal.test.tsx
@@ -0,0 +1,111 @@
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import ChangeInvoiceTokenModal from "@/components/ChangeInvoiceTokenModal";
+import { buildConvertInvoiceTokenTransaction, type Invoice } from "@/utils/soroban";
+
+const runTransaction = vi.fn();
+const addToast = vi.fn(() => "toast-id");
+const updateToast = vi.fn();
+
+vi.mock("@/hooks/useTransaction", () => ({
+  useTransaction: () => ({
+    isPending: false,
+    runTransaction,
+  }),
+}));
+
+vi.mock("@/context/ToastContext", () => ({
+  useToast: () => ({ addToast, updateToast }),
+}));
+
+vi.mock("@/hooks/useApprovedTokens", () => ({
+  useApprovedTokens: () => ({
+    tokens: [
+      { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" },
+      { contractId: "CEURC", name: "Euro Coin", symbol: "EURC", decimals: 7, iconLabel: "EU" },
+    ],
+    tokenMap: new Map([
+      ["CUSDC", { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" }],
+      ["CEURC", { contractId: "CEURC", name: "Euro Coin", symbol: "EURC", decimals: 7, iconLabel: "EU" }],
+    ]),
+    defaultToken: { contractId: "CUSDC", name: "USD Coin", symbol: "USDC", decimals: 7, iconLabel: "US" },
+    isLoading: false,
+    error: null,
+  }),
+}));
+
+vi.mock("@/utils/soroban", async () => {
+  const actual = await vi.importActual<typeof import("@/utils/soroban")>("@/utils/soroban");
+  return {
+    ...actual,
+    buildConvertInvoiceTokenTransaction: vi.fn().mockResolvedValue("tx"),
+  };
+});
+
+const invoice: Invoice = {
+  id: 12n,
+  status: "Pending",
+  freelancer: "GFREELANCER",
+  payer: "GPAYER",
+  amount: 10_000_000n,
+  due_date: 1_900_000_000n,
+  discount_rate: 300,
+  token: "CUSDC",
+};
+
+describe("ChangeInvoiceTokenModal", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    runTransaction.mockImplementation(async (builder: () => Promise<unknown>) => {
+      await builder();
+      return { txHash: "hash123" };
+    });
+  });
+
+  it("shows the current token, allowlisted token selector, and denomination warning", () => {
+    render(
+      <ChangeInvoiceTokenModal
+        invoice={invoice}
+        submitter="GFREELANCER"
+        onClose={vi.fn()}
+        onSuccess={vi.fn()}
+      />
+    );
+
+    expect(screen.getByRole("heading", { name: "Change Token" })).toBeInTheDocument();
+    expect(screen.getByText("This changes the currency your invoice is denominated in.")).toBeInTheDocument();
+    expect(screen.getByText("USDC")).toBeInTheDocument();
+    expect(screen.getByRole("combobox", { name: /new token/i })).toHaveValue("CEURC");
+  });
+
+  it("builds and submits convert_invoice_token through useTransaction", async () => {
+    const onClose = vi.fn();
+    const onSuccess = vi.fn();
+
+    render(
+      <ChangeInvoiceTokenModal
+        invoice={invoice}
+        submitter="GFREELANCER"
+        onClose={onClose}
+        onSuccess={onSuccess}
+      />
+    );
+
+    fireEvent.click(screen.getByRole("button", { name: "Change Token" }));
+
+    await waitFor(() =>
+      expect(buildConvertInvoiceTokenTransaction).toHaveBeenCalledWith({
+        submitter: "GFREELANCER",
+        invoiceId: 12n,
+        newToken: "CEURC",
+      })
+    );
+    expect(runTransaction).toHaveBeenCalledTimes(1);
+    expect(updateToast).toHaveBeenCalledWith(
+      "toast-id",
+      expect.objectContaining({ type: "success", txHash: "hash123" })
+    );
+    expect(onSuccess).toHaveBeenCalledWith("CEURC");
+    expect(onClose).toHaveBeenCalled();
+  });
+});
diff --git a/src/hooks/useTransaction.ts b/src/hooks/useTransaction.ts
new file mode 100644
index 0000000..aae4305
--- /dev/null
+++ b/src/hooks/useTransaction.ts
@@ -0,0 +1,45 @@
+"use client";
+
+import { useState } from "react";
+import { useWallet } from "@/context/WalletContext";
+import { submitSignedTransaction } from "@/utils/soroban";
+import type { Transaction } from "@stellar/stellar-sdk";
+
+export type TransactionState = "idle" | "preparing" | "signing" | "success" | "error";
+
+export function useTransaction() {
+  const { signTx } = useWallet();
+  const [state, setState] = useState<TransactionState>("idle");
+  const [error, setError] = useState<string | null>(null);
+
+  const runTransaction = async (buildTransaction: () => Promise<Transaction>) => {
+    setState("preparing");
+    setError(null);
+
+    try {
+      const tx = await buildTransaction();
+      setState("signing");
+      const result = await submitSignedTransaction({ tx, signTx });
+      setState("success");
+      return result;
+    } catch (transactionError) {
+      const message = transactionError instanceof Error ? transactionError.message : "Transaction failed.";
+      setError(message);
+      setState("error");
+      throw transactionError;
+    }
+  };
+
+  const reset = () => {
+    setState("idle");
+    setError(null);
+  };
+
+  return {
+    state,
+    error,
+    isPending: state === "preparing" || state === "signing",
+    runTransaction,
+    reset,
+  };
+}
diff --git a/src/utils/soroban.ts b/src/utils/soroban.ts
index 02aa7a8..3bcd3f3 100644
--- a/src/utils/soroban.ts
+++ b/src/utils/soroban.ts
@@ -1016,6 +1016,44 @@ export async function buildApproveUsdcTransaction(args: {
   return buildApproveTokenTransaction(args);
 }
 
+// ─── Write: invoice token conversion ─────────────────────────────────────────
+
+export async function buildConvertInvoiceTokenTransaction({
+  submitter,
+  invoiceId,
+  newToken,
+}: {
+  submitter: string;
+  invoiceId: bigint;
+  newToken: string;
+}) {
+  const account = await server.getAccount(submitter);
+  const tx = new TransactionBuilder(account, {
+    fee: BASE_FEE,
+    networkPassphrase: NETWORK_PASSPHRASE,
+  })
+    .addOperation(
+      Operation.invokeContractFunction({
+        contract: CONTRACT_ID,
+        function: "convert_invoice_token",
+        args: [
+          nativeToScVal(invoiceId, { type: "u64" }),
+          Address.fromString(newToken).toScVal(),
+        ],
+      })
+    )
+    .setTimeout(60)
+    .build();
+
+  const simulated = await server.simulateTransaction(tx);
+  if (!rpc.Api.isSimulationSuccess(simulated)) {
+    const message =
+      "error" in simulated ? simulated.error : "Unable to simulate invoice token conversion.";
+    throw new Error(`Simulation failed: ${message}`);
+  }
+  return rpc.assembleTransaction(tx, simulated).build();
+}
+
 // ─── Write: generic signed transaction dispatcher ─────────────────────────────
 
 export async function submitSignedTransaction({