Does the written loan contract remain legally valid with the verbal condition of working extra hours without pay?

If an increase is stipulated in a loan, it is a usurious and prohibited loan, whether the increase is in kind, or a benefit, or free labor, and whether the condition is written or verbal. Muslims have unanimously agreed on the prohibition of stipulating an increase in loans, even if it is a simple benefit. It is forbidden to accept a gift or benefit from the borrower during the loan period if it was not customary before that, because it is considered usury. Therefore, a loan that stipulates free extra working hours is a usurious and prohibited loan.