Program it to be 100% discount, and rounding up, and then whatever you ring up, if you want to void the entire transaction, just hit % and then cash out, and the entire transaction will come through as 0.00. Not quite the same as voiding, but it does work well enough. You do lose the ability to do another quick discount, but it’s worth it.
https://www.amazon.com/Casio-PCR-T280-Electronic-Cash-Register/dp/B002TXSR92

