diff --git a/database/migrations/086_sms_balance_credit_units.sql b/database/migrations/086_sms_balance_credit_units.sql
index 0a3a9b9..a944af5 100644
--- a/database/migrations/086_sms_balance_credit_units.sql
+++ b/database/migrations/086_sms_balance_credit_units.sql
@@ -1,11 +1,12 @@
--- Convert prepaid SMS balance from IRT to credit units.
+-- Convert prepaid SMS balance from IRT to credit units (idempotent).
 -- Previous billing: 400 IRT per 64-char segment.
 -- New model: balance is SMS credits (1 credit = 1 segment); top-up at 500 IRT/credit.
--- Preserve existing prepaid capacity: IRT / 400 → credits.
+-- Only convert rows that still look like IRT balances (>= 100_000). Already-converted
+-- credit balances are below that and are left unchanged on re-run.
 
 UPDATE businesses
 SET sms_balance = sms_balance / 400
-WHERE sms_balance > 0;
+WHERE sms_balance >= 100000;
 
 COMMENT ON COLUMN businesses.sms_balance IS
   'Prepaid SMS credit units. 1 unit = one 64-character segment per recipient. Top-up adds floor(IRT / 500) credits.';
diff --git a/src/auth/sms-billing.service.ts b/src/auth/sms-billing.service.ts
index 3d1725f..914c99c 100644
--- a/src/auth/sms-billing.service.ts
+++ b/src/auth/sms-billing.service.ts
@@ -1,7 +1,7 @@
 import { BadRequestException, Injectable, NotFoundException } from '@nestjs/common';
 import { PrismaService } from '../prisma/prisma.service';
 import { TenantService } from '../tenant/tenant.service';
-import { calculateSmsCredits, irtToSmsCredits } from './sms-billing';
+import { calculateSmsCredits, irtToSmsCredits, SMS_PURCHASE_PRICE_PER_CREDIT_IRT } from './sms-billing';
 
 @Injectable()
 export class SmsBillingService {