Detective/Retardation

The good news is that this happens even to professional cashiers. But rarely. But it does happen. And that's why there are rituals for handing out money.
Count the money, transferring it from hand to the table or from one hand to the other, TWICE. If the amount is the same both times, give it to the recipient.
And, of course, recount the money when receiving it.

I would add "don't keep money in a stack on a shelf." Money should be hidden as little as possible so that various couriers can't "accidentally teleport" it into their palms. I understand that in this case, you mainly suspect your own absentmindedness, Revan, but still... Trust, but verify.
Although you say later that you put it in the cash register... Well, then, never mind, I was wrong.