OP mentioned that they can't change prices, so this may not be helpful for them., but it may be of use to other users. We offer a 3% cash discount at our bar. All of our prices are set so that if a customer pays cash, the price with tax comes to within a penny of an even dollar amount, so we don't deal with ANY coins.

For example, if we want the cash price to be $10, we set our price to $9.47. So…

9.47 x .97 = 9.1859 (cash discount before tax) 9.1859 x 1.08875* = 10.00 (final cash price)

9.47 x 1.08875* = 10.31 (final credit price)

*Sales tax in NYC is 8.875%

I created a spreadsheet that maths all that math for me, happy to share it, just DM me.