No matter what, any agreements that you make with a tenant should be put in writing.
This applies to leases, addendums, amendments, and the most basic of promises.
Your rental property is a business, with real money and equity involved – so treat it that way.
Leases should ALWAYS be a signed written document/contract – no exceptions. For everything else, some sort of signed paper is best, but a thread of emails also works well.
Verbal leases and agreements are valid but difficult to prove in a court of law. If you have to make a verbal agreement, use your phone to record the audio conversation. It’s sad to say that a handshake doesn’t mean as much as it use to.