What should the recipient do with money disbursed to him by mistake from the United Nations that was not deducted, and should he give it as charity or keep it in anticipation of it being claimed in the future?

The money whose owners are known must be returned to them if possible. Otherwise, you may give it as charity, while still guaranteeing it, should they demand it and not approve of your action. You may also keep it as a debt to be deducted from you, or dispose of it for your benefit while guaranteeing its equivalent, especially if you know yourself to be capable of repayment upon demand. If you despair of returning it to its owners, then it must be spent on the general welfare of Muslims, and it is not permissible to withhold it.