24 CFR §1005.723

1. (a)A Servicer must have a written policy on how it handles Partial Payments, in compliance with this section and that policy shall be readily available to the public.
2. (b)Upon receipt of a Partial Payment, a Servicer must provide the Borrower a copy of the Servicer's written Partial Payment policy and a letter explaining how it will handle the received Partial Payment. The Servicer may:
   1. (1)Accept a Partial Payment and either apply it to the Borrower's account;
   2. (2)Identify it with the Borrower's account number and hold it in a trust account pending disposition; or
   3. (3)Return the Partial Payment(s) to the Borrower.