If the jeweler usually gives your MIL really good deals, then ask if she can accompany you when you meet up with him. I had similar situations in the past with my mom's jeweler. He tends give a better price whenever she's around. Maybe it's the way she looks at him whenever he quotes a price that's she doesn't like.
If there is a cost difference in the new ring, then maybe get something to make up the difference. I find that jewelers do not like to give back the difference when you are doing a trade-in.
I would definitely purchase something rather than have him give me any money back. I hope it's that easy.
I actually have a necklace I want made so maybe if all goes well I'll have that done. Thank you ~ you just made me think about that, I'll bring the stone I want set in the necklace when I meet with him.
I really appreciate everyone's thoughts!