I agree this would be useful, when I train clients on using the system some of them don’t understand (or later forget) those are $ amount buttons. Adding the currency symbol would solve that. Plus most other systems on the market I have seen is like Cake where it has the currency symbol.
I had someone ask for this before. Also had a request for round up to next £5, so if the amount was £5.23, the button would show £10. I thought about doing my own automation for it but eventually decided against it. This could be the same functionality as you request, just you provide a value for rounding up, so can be 1, 5, etc.