This tip truly applies to any interaction you make when acting in a professional capacity (whatever your profession may be) or when discussing anything that may result in a legally binding contract.
Agreements can be entered into in two ways, either by a written agreement or an oral agreement.
When commencing any type of negotiation or entering into an agreement for anything, the most important advice I can provide is to keep detailed notes. Try to communicate with one another electronically, whether through email or text, to maintain a record of your discussions. If you opt to write personal notes, it is just as sufficient. Just remember to date them and include as much detail as possible.
Keeping detailed records can help refresh your memory and also assist you if your interaction results in litigation.