How to request for extended stay after the house closing

[Pete Tam]

Hello,

One of my relative is selling his house on his own (Sale by owner) and his home closing is coming up soon. He requested buyer that he would like to stay extra 2 more weeks before he moves out and the buyer agrees to it, but he is wondering if he should get something in writing and get it signed by both buyer and seller, and may be buyer's agent signature too. 

Any recommendation? Any example letter that I can share with him?