You shouldn't refuse to give back money you owe.
Frank owes money to his friend who just found out that he has just received his paycheck but it's only enough to cover his bills.
Frank wants to avoid getting any late fees.
Frank pays the money back to the friend and uses his credit card to pay some of his bills.
Frank claims that he has not actually been paid yet to keep the money.
Frank's friend learns that he will repay loans and it increases his trust.
Frank's friend loses trust in him and will not help him in the future.