Does the amount presented as an end-of-service bonus from the United Nations, which includes "interest $639.90," contain usury (riba)? If so, what should be done with it, and is it permissible to give it away in charity?

If a sum of money is sent to you, you must first ask the sender the reason for sending it. If it is rightfully yours or a gift, then there is no objection to you owning it. However, if it consists of usurious interest due to a delay in payment, then you are only permitted to take your original rightful amount. You must dispose of the interest by spending it on general Muslim welfare or on the poor and needy, not with the intention of charity.