Receiving gifts is hard. It is a learned skill, and does not come naturally to most people. You need to get out of your head, and into hers: This is her expressing something to you that is hard. Not easy. Honor where the gift is coming from, not the gift itself. Receive it fully and generously and with your heart. Appreciate the giver. Anything less is dishonoring the gift, and disempowering the giver... devaluing their gift, at the very moment they are most vulnerable and exposed. - Then put the money aside. Months later, you can spend it on her. Take her out to dinner. Get her that thing she wants/needs that she wouldn't buy for herself. Never tell her where the money came from. You both win.
This is probably the right answer. It's still weird in my head because it's effectively absorbing the cash into my budget and then deciding to take it back out of a budget months later. I do think there's some responsibility on the part of the gift giver to understand how their gift impacts the recipient. It's why pets are almost always a bad gift. I also get tripped up putting myself in her head because I don't grasp why she thinks cash is a great gift. It was when I was 15 or 20, but at 38 when she's 71, it isn't. It feels kind of infantilizing.
I hear ya... but you are still 15 in her head. Especially now that she's 71, and her perceptions of the world are narrowing down, new memories are harder to store/recall, and she spends a lot of time in her old memories. The gift isn't about you; it's about her. Accepting it generously is going to make her happy. And that's just a nice thing to do for another human being, regardless of your relationship with them.