Many years ago, I represented a client in a trial involving an alleged oral agreement with a business partner. We won the case, but only after a trial and a great deal of angst and expense. The client bought me a teeshirt afterwards that said "Put It In Writing".
A recent decision of the Massachusetts Superior Court illustrates the importance of all such agreements being in writing.
In a case involving a minority shareholder that left to form her own company, Judge Patrick Brady (a very experienced trial court judge), ruled that even though the minority shareholder did owe a fiduciary duty to the company she left, in the absence of an employment agreement or a non compete agreement, there was nothing that prevented her from leaving and using her actuarial skills to earn a living. The company from which she departed had alleged that the resignation left it unable to service many of it's clients in the public sector. Judge Brady went on to further rule that the company could engage in efforts, including hiring someone new, to get that work done.
I could not even to begin to chronicle the number of times I have been in court over 32 years because someone in business came to me and claimed they had an "oral" agreement with a business partner or another party; what we call "he said/she said" cases.
If you have an agreement; put it in writing and don't leave things to chance.