By including a per diem amount in the letter, the parties will not have to execute another payoff letter if the termination date. The statement includes information and formulas to calculate on a per diem basis the payoff amount after the date it is issued.Discover more about per diem interest—the daily interest on a loan that occurs outside of the standard repayment period. A 10 day payoff is crucial to getting your old loans paid off during a refinance. Learn how it works from start to finish. It typically details the principal balance, interest due as of that date, per diem, any late fees, etc. The new interest calculation applies when an FHA loan with a Note date of January 21, 2015 or after, is paid in full.