Handshake deal

A handshake deal is a verbal commitment to a transaction. For the deal or agreement to become binding: one must witness the deal take place, have a follow up email or some form of communication to the other party, have any correspondence or other documentation that could be used as "evidence" in court or other legal needs, and/or begin to perform to the agreements in the deal.

Example 1:

Example 2:

Example 3:

...
Wikipedia