ACCOUNT PROBLEM

My father has an account with Mr Price, they agreed for him to have R5000 credit it shouldn't increase or decrease. He pays R750 per month as per agreement and the last time he took clothes was in January this year. The buying power stays at not more than R2000 yet we agreed on R5000. In January the balance was R4999 yet in May it was R3000. How is the balance R3000 when R750×4=R3000. R4999-R3000=R1999 meaning that the balance was supposed to be R1999. Mr Price please fix this