We have enough sales that we can absorb the loss. That’s one advantage for working for a large corporation. Of course, we are also fortunate that 1) the company is willing do this and 2) local management backs up local workers. Neither of those is a given.
Also - invariably when this comes up, most other bystanding customers fully support us not taking slobbered on cash.

Of course, he just stuck those icky bills back in his wallet and spent them somewhere else.
Yes. Well, the world is not perfect. We can protect our staff and other customers. Maybe if other businesses refuse cash after people contaminate bills shitheads like him will get the message that this is completely unacceptable.

The better action would have been: Call Store Security over on a CodeRED, and when they arrive in their full space suits, they confiscate those poisoned bills and burn them on the spot!
Ah… fantasies…!
Yesterday, we had an old lady who decided she needed more plastic bags. So she loads up a hand with spit to peel them off the carousel. She is told “please don’t do that”, handed a clean bag, and the pile she touched is taken off the hook and thrown out.
Five minutes later she’s at another carousel about to do the same thing. I said “Ma’am, I’m sorry, we can’t allow that. If you need help with the bags please ask. Here’s a bag for you.” She glared at me, throw the bag on the floor, and stormed off. Well, tottered off, but it was an angry old-age slowed walk.
One thing I have noticed is that these people are, so far older White people. Every single one. And the excuse, if one is given, is “I’ve been doing this all my life”. Well, fucking change your disgusting, disease-spreading habits! Working retail I get to see both White Privilege and Wealth Privilege on full display, sometimes even intersecting.
And yeah, people get offended - you’re saying I’m diseased! Well, boo-fucking-hoo! You have cooties just like everyone else! These assholes would be the first to take offense if someone else did this to them. Fuck 'em.