What is the ruling on an employee taking a 15% increase on their salary as compensation for delayed payment, knowing that they work in a public institution?

This increase is impermissible, because the salaries have become a debt owed by the institution, and any increase on a debt in exchange for delaying its payment is considered prohibited usury (riba). Sheikh Ulaish stated in "Fath al-Ali al-Malik" that the debtor's commitment to an increase on a debt in exchange for delaying its payment is void and explicit usury, whether the increase is of the same type as the debt or different, and whether it is a specific thing or a benefit.