Improper profits: Refers to the benefits gained by others without any legal or contractual basis. The profitee is the debtor and has the obligation to return the improper profits; the injured person is the creditor and enjoys the requested benefit. Right to return improper profits. According to this definition, Which of the following are unjust gains:. ['Handling fees charged by banks when handling inter-bank deposits and withdrawals.', 'Penalty paid by Manager Liu for excess baggage while flying.', 'Xiao Zhang won 200 yuan with relatives at home during the Spring Festival.', 'Xiaomei, the cashier at the mall, mistakenly found 100 yuan as 10 yuan to customers.']