RCS charging interest on closed account

very disappointed with this company and how they handle their business. I got a settlement amount on 12/12/2023 and paid the same day. they sent me a paid up letter on 18/12/2023. they also informed me that there's moneys due to me and I must send a copy of my ID and proof of banking for them to refund me. I sent them the requested documents on 03/01/2024 and again on 09/01/2024 without any response. I received a statement today 11/12/2024 and learnt that they charged me interest on this closed account. so instead of RCS refunding me, they deduct interest on this amount that they should be refunding. I want this rectified