diff --git a/src/dstack/_internal/server/background/pipeline_tasks/jobs_submitted.py b/src/dstack/_internal/server/background/pipeline_tasks/jobs_submitted.py
index 9e93e7b2a..5f5ac0eeb 100644
--- a/src/dstack/_internal/server/background/pipeline_tasks/jobs_submitted.py
+++ b/src/dstack/_internal/server/background/pipeline_tasks/jobs_submitted.py
@@ -98,6 +98,7 @@
     get_instance_offer,
     get_instance_provisioning_data,
     switch_instance_status,
+    try_atomic_busy_blocks_increment,
 )
 from dstack._internal.server.services.jobs import (
     check_can_attach_job_volumes,
@@ -575,6 +576,22 @@ async def _apply_assignment_result(
                 return
 
             instance_model, current_offer = current_instance_offers[0]
+            if not await try_atomic_busy_blocks_increment(
+                session=session,
+                instance_id=instance_model.id,
+                blocks=current_offer.blocks,
+            ):
+                await _reset_job_lock_for_retry(session=session, item=item)
+                return
+
+            # Refetch instance to get updated busy_blocks from the atomic update
+            res = await session.execute(
+                select(InstanceModel)
+                .where(InstanceModel.id == instance_model.id)
+                .options(joinedload(InstanceModel.volume_attachments))
+            )
+            instance_model = res.unique().scalar_one()
+
             _assign_instance_to_job(
                 session=session,
                 job_model=job_model,
@@ -880,7 +897,6 @@ def _assign_instance_to_job(
     job_model.job_runtime_data = _prepare_job_runtime_data(offer, multinode).json()
 
     switch_instance_status(session, instance_model, InstanceStatus.BUSY)
-    instance_model.busy_blocks += offer.blocks
     events.emit(
         session,
         (
diff --git a/src/dstack/_internal/server/background/scheduled_tasks/submitted_jobs.py b/src/dstack/_internal/server/background/scheduled_tasks/submitted_jobs.py
index eaa452380..f65b3fff0 100644
--- a/src/dstack/_internal/server/background/scheduled_tasks/submitted_jobs.py
+++ b/src/dstack/_internal/server/background/scheduled_tasks/submitted_jobs.py
@@ -93,6 +93,7 @@
     format_instance_blocks_for_event,
     get_instance_provisioning_data,
     switch_instance_status,
+    try_atomic_busy_blocks_increment,
 )
 from dstack._internal.server.services.jobs import (
     check_can_attach_job_volumes,
@@ -1079,6 +1080,12 @@ async def _assign_existing_instance_to_job(
 
     instances_with_offers.sort(key=lambda instance_with_offer: instance_with_offer[0].price or 0)
     instance, offer = instances_with_offers[0]
+    if not await try_atomic_busy_blocks_increment(
+        session=session, instance_id=instance.id, blocks=offer.blocks
+    ):
+        job_model.instance_assigned = False
+        return
+
     # Reload InstanceModel with volume attachments
     res = await session.execute(
         select(InstanceModel)
@@ -1087,7 +1094,6 @@ async def _assign_existing_instance_to_job(
     )
     instance = res.unique().scalar_one()
     switch_instance_status(session, instance, InstanceStatus.BUSY)
-    instance.busy_blocks += offer.blocks
 
     job_model.instance = instance
     job_model.used_instance_id = instance.id
diff --git a/src/dstack/_internal/server/services/instances.py b/src/dstack/_internal/server/services/instances.py
index d54ec8b68..7fd373b99 100644
--- a/src/dstack/_internal/server/services/instances.py
+++ b/src/dstack/_internal/server/services/instances.py
@@ -5,7 +5,7 @@
 from typing import Dict, List, Literal, Optional, Union
 
 import gpuhunt
-from sqlalchemy import and_, exists, false, or_, select
+from sqlalchemy import and_, exists, false, or_, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import joinedload, load_only
 
@@ -63,6 +63,29 @@
 logger = get_logger(__name__)
 
 
+async def try_atomic_busy_blocks_increment(
+    session: AsyncSession,
+    instance_id: uuid.UUID,
+    blocks: int,
+) -> bool:
+    """
+    Atomically increment busy_blocks if the instance has capacity.
+    Returns True if the update affected a row, False otherwise.
+    """
+    res = await session.execute(
+        update(InstanceModel)
+        .where(
+            InstanceModel.id == instance_id,
+            or_(
+                InstanceModel.total_blocks.is_(None),
+                InstanceModel.busy_blocks + blocks <= InstanceModel.total_blocks,
+            ),
+        )
+        .values(busy_blocks=InstanceModel.busy_blocks + blocks)
+    )
+    return res.rowcount > 0
+
+
 def switch_instance_status(
     session: AsyncSession,
     instance_model: InstanceModel,