From what I have made out of your query it appears that you did in fact withdrew and spent money which legally did not belong to you. However you did so under the mistaken belief that it was yours to withdraw. Since you have already asked them for granting extra time to you to return the money it amounts to an admission on your part. So I suggest you respond in writing to their legal notices and request for a fixed period of time by which you can transfer the money. Also point out in your reply that it is an error on their part also for not checking the amount before transferring. Say that be that as it may you will still pay back what they are legally entitled to. Please do not delay this as they will inflate the amount and also demand interest from you.