I have to admit it does sound as if she is motivated by money. At the same time I know how you feel regarding the betrayal by your MIL. My FMIL had a falling out last Easter and haven't spoken since. she would rather appease her wayward son than stand up to him and let her feelings be known.
She knew how hurt and angry I was and she just kept telling me "You'll never be able to trust him again". Probably true but not a call that I think she should have been trying to make. I say take a couple of days, calm down and then ask your MIL to have a chat. I would let her know that you understand this is her business but that her offer hurt you to the core.
Hugs, kat
Me-53(and learning!) S24, S21, D18, D17 Just keep swimming, Just keep swimming. Dory