MM, if you want to work well together seek to understand. Neither of you is hearing the other. You're just posturing. Set a good time to talk and start by asking him to tell you about his idea to purchase new inventory. Be truly interested and enthusiastic if you can muster it. Really listen. The purchase might make sense. You can remind him of the original business plan that you agreed on. Let him know that you would like to understand the reason for this new approach. If it seems detrimental to the business then voice your concerns. You can then work toward a compromise. Be friendly about it. You both seem to back one another into a corner and the other comes out swinging. It's not working.
