A couple of years ago, I enrolled for a course. I paid the fees. But later for some legitimate reasons I withdrew and the organisation returned the money to me. But they made a..

Assalamu-alaikum,

A couple of years ago, I enrolled for a course. I paid the fees. But later for some legitimate reasons I withdrew and the organisation returned the money to me. But they made a mistake and returned more than the fee  i paid for. I informed them in writing. Since I did not know how to return it. They acknowledge the mistake and wrote me that they will get in touch with me to inform how may i return their money. nothing happened. I made another attemp and i've been told that they passed the issue to appropriate department and they will pursue from there. I was waiting for some time. nothing happened. Meanwhile I spent the money. Now, I'm intending to go and return the money to them. what shoud I do? shuld I return the money in exact amount of extra payment that I received or I should be cautious and pay some more? One more question, I'm afraid of the issue. so I'm intending to return it as donation since they have an avenue to receive donations from public. will it be fine to do so as anonymous? Jazakumullah khair