Is it permissible for me to keep money that a colleague sent me for an appointment, then demanded it back two months later, claiming it was a trust or for a legitimate purpose, knowing that I have already spent the money?

If your friend grants you an amount of money and then regrets it and wishes to retract, he is not entitled to do so after you have taken possession of it and secured it, due to the Prophet’s (peace and blessings be upon him) saying: "The one who retracts his gift is like the one who retracts his vomit." His claim thereafter that he gave you the amount as a trust, or a partnership, or a loan is not to be accepted, according to the preponderant opinion, and your word is to be taken, due to the apparent meaning of the wording and the circumstantial evidence. In case of dispute, the Shariah judiciary is the decisive authority.