Threatening letters

I was receiving a social grant for my child. I got a permanent job in 2015 march and in april i wrnt to sassa to stop the grant as i was permanently employed. They said i withdrawn the R330. I received in march therefore i need to repay it as i only went to their offices in april. I didnt hesitate and i paid the R330 into the account number i was given by them. Within few months i paid i received a letter from sassa that i didnt pay, i phoned the joburg offices and the lady i spoke to said i should ignore the letter as the payment will reflect on the system. Its been a year now and to my surprise i received another letter yesterday 16.03.2016 that i need to pay or further steps will be taken against me. I mean how could i not pay the R330 while im the one who notified sassa im permanent and again the lady said it will reflect on the system. I don't have proof of payment anymore as the receipt was small and was told not to worry. I have phoned the mamelodi branch and the guy i spoke to told me to come to their offices as there is nothing he can do. How do i have to go there, take a day off at work for something that was not my fault while i was ensured i will be cleared on system