When someone offers to help you make a down payment on a house, your lender will require a gift letter for a mortgage. Here's what it includes.A mortgage gift letter is a document that helps satisfy the requirement that a borrower's down payment funds come from legitimate sources. Donate to the Greater Chicago Food Depository and help end hunger in Cook County. Fill out our food industry donation form today to show your support. In this post, I'm breaking down how to write a donation letter.