A letter agreement is a written legal document that is made between two parties who want a simple way to make their discussions of a transaction official. A letter of agreement is an agreement between two parties that puts the terms of the agreement in writing as a means of resolving later disputes that may arise.Establish terms of a working relationship. A purchase agreement letter between seller and buyer is created when two parties come together, where one party desires to purchase a piece of property. A. During and at the completion of the Residency Period, Sponsor agrees to: 1. Provide its professional services as administrator of the this program,. 2.