Since last year I requested to be refunded the money that got into my previous account that was closed since 2022,they have been sending me from pillar to post

Since last year I requested to be refunded the money that got into my previous account that was closed since 2022,they have been sending me from pillar to post. They asked me to send the proof from where the money was from, it was done,now they are asking me to write a letter to authorizing the money to be refunded to the person who paid it,while from the onset I told them the money was deposited by a mistake and I even gave them a proof of who deposited it, she even sent the proof of bank statement as requested but now the scenario are changing every day.