Holy heck! Talk about going above and beyond! Time to just say no in my opinion. It may be challenging to get your partner to accept this and back you up, but it is actually his boundary to make, since she is his mother. No doubt she will make a huge fuss, but you will be well within your rights to just tune her out.
i don't care how much money you have or don't have. That is not the issue. Her greed is the issue!