The person who asks for a written agreement is protecting the relationship from a future misunderstanding.

Requests to put things in writing are often heard as distrust, especially between friends or family. The opposite is usually true: writing it down means neither of you will have to rely on memory when money is involved and the details have gone soft. Most falling-outs over arrangements are not about dishonesty but about two honest people remembering differently.

When you agree something that involves money, time or property, write a short version and send it to the other person. Keep it plain and friendly, and treat it as care for the friendship rather than a doubt about it.