Is a business partnership entered into via a verbal agreement valid

This depends on the partnership law in your state. I can tell you that in NY (since I do not practice in MT) and in many other states a partnership can be oral in nature. However, bear in mind that the oral agreements tend to suffer greatly when one partner wants to enforce a specific action, especially a buy out.

I hope this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.