Is it Islamically obligatory to return sums of money that a person paid as a gift, then later demanded back, claiming they were a loan, knowing that the beneficiary would not have accepted them as a loan?

Linguistically, a loan is: what you give to receive back. Legally, it is: the transfer of ownership of something on the condition that its equivalent be returned, or any wording that conveys this meaning. If someone gives you money, whether cash, bills, or other forms, and does not ask for its return at the time of payment, you are not obligated to return it immediately.