Loan overpayment.

I had a loan with fnb where the last payment would b 25 May 2020, In March I requested a settlement letter R3 363.12 was quoted and R3 514.81 in April was quoted, The Payer accidentally paid R4 914.12, that means fnb owes me R1 399.21, every time when I call no 1 answers and 2 Complaints were lodged with them and all I got was an sms confirming the matter will be looked at, if I was the 1 owing them i would be threatened with legal actions and every other threat available on in the world, they are too relaxed because the debt is on their side.