diff --git a/src/backend/src/services/reimbursement-requests.services.ts b/src/backend/src/services/reimbursement-requests.services.ts
index a5c383883b..8e5486527a 100644
--- a/src/backend/src/services/reimbursement-requests.services.ts
+++ b/src/backend/src/services/reimbursement-requests.services.ts
@@ -699,13 +699,19 @@ export default class ReimbursementRequestService {
     if (reimbursementRequest.organizationId !== organization.organizationId)
       throw new InvalidOrganizationException('Reimbursement Request');
 
+    const existingWithSaboNumber = await prisma.reimbursement_Request.findFirst({
+      where: { saboId: saboNumber, organizationId: organization.organizationId }
+    });
+    if (existingWithSaboNumber) {
+      throw new HttpException(400, 'This SABO number is already assigned to another reimbursement request.');
+    }
+
     const reimbursementRequestWithSaboNumber = await prisma.reimbursement_Request.update({
       where: { reimbursementRequestId },
       data: {
         saboId: saboNumber
       }
     });
-
     return reimbursementRequestWithSaboNumber;
   }
 
diff --git a/src/backend/tests/unit/reimbursement-requests.test.ts b/src/backend/tests/unit/reimbursement-requests.test.ts
index 0717511bc0..b1d21a0bc4 100644
--- a/src/backend/tests/unit/reimbursement-requests.test.ts
+++ b/src/backend/tests/unit/reimbursement-requests.test.ts
@@ -785,4 +785,28 @@ describe('Reimbursement Requests', () => {
       ).rejects.toThrow(new NotFoundException('Reimbursement Product Other Reason', 'bad id'));
     });
   });
+
+  describe('Setting the SABO number on a reimbursement request', () => {
+    test('Fails when SABO number is already assigned to a request', async () => {
+      await ReimbursementRequestService.setSaboNumber(
+        reimbursementRequest.reimbursementRequestId,
+        'SABO-001',
+        createdUser,
+        org
+      );
+      await expect(
+        ReimbursementRequestService.setSaboNumber(reimbursementRequest.reimbursementRequestId, 'SABO-001', createdUser, org)
+      ).rejects.toThrow(new HttpException(400, 'This SABO number is already assigned to another reimbursement request.'));
+    });
+
+    test('Successfully sets the SABO number', async () => {
+      const result = await ReimbursementRequestService.setSaboNumber(
+        reimbursementRequest.reimbursementRequestId,
+        'SABO-001',
+        createdUser,
+        org
+      );
+      expect(result.saboId).toEqual('SABO-001');
+    });
+  });
 });
diff --git a/src/frontend/src/apis/finance.api.ts b/src/frontend/src/apis/finance.api.ts
index bcc112c199..604ea39ea2 100644
--- a/src/frontend/src/apis/finance.api.ts
+++ b/src/frontend/src/apis/finance.api.ts
@@ -363,7 +363,7 @@ export const getPendingAdvisorList = () => {
  * @param saboNumber the SABO number to set
  */
 export const setSaboNumber = async (requestId: string, saboNumber: string) => {
-  axios.post(apiUrls.financeSetSaboNumber(requestId), {
+  return axios.post(apiUrls.financeSetSaboNumber(requestId), {
     saboNumber
   });
 };
diff --git a/src/frontend/src/pages/FinancePage/ReimbursementRequestDetailPage/AddSABONumberModal.tsx b/src/frontend/src/pages/FinancePage/ReimbursementRequestDetailPage/AddSABONumberModal.tsx
index a56c4bf7ee..40c4d13554 100644
--- a/src/frontend/src/pages/FinancePage/ReimbursementRequestDetailPage/AddSABONumberModal.tsx
+++ b/src/frontend/src/pages/FinancePage/ReimbursementRequestDetailPage/AddSABONumberModal.tsx
@@ -34,12 +34,12 @@ const AddSABONumberModal = ({ modalShow, onHide, reimbursementRequestId }: AddSA
   const onSubmit = async (data: { saboNumber: string }) => {
     try {
       await setSaboNumber(data);
+      onHide();
     } catch (error: unknown) {
       if (error instanceof Error) {
         toast.error(error.message);
       }
     }
-    onHide();
   };
 
   return (