Ah you buy just the right amount of monero at an exchange (plus a bit for fees) and send that to the person you are paying? Could also keep a balance in a stablecoin and swap what you need to monero then.
Discussion
no, you can't. at least not legally without tax in most countries.
if you don't want to pay tax, there needs to be no capital gain, so the exchange rate has to be the same. you have to buy and use all the monero for payment, holding zero at any moment.