Should I go behind my mother's back?

My grandmother recently lost her drivers license due to a minor accident (She’s 80). I was thinking of asking her if I could buy her vehicle from her. I’ve got cash. I had been worrying about what I was going to do about my car…it has over 100 thousand miles on it and I have horrible credit so I know I would never be able to buy one myself when my car bites the dust, which it will soon. Anyway….I thought this would be a win/win situation. I text my mother to see what she thought before I brought it up to my grandmother and she replies “Ive talked her out of selling…but if she does I have first dibs on it. Sorry”. My mother has 3 vehicles AND the credit to buy a vehicle. When my car dies on me…Im screwed!!! So my question is, should I pretend like I haven’t spoken with my mother, and talk to my grandma about my situation?