Well, that was a peculiar and frustrating experience in balancing diplomacy and paying the right price for groceries to the multinational corporation. Realized as I stepped away from the till that I had the wrong change in hand, but there was a logic to the error, so it took me a sec. Scribbled some numbers in the pad that I thought would make it clear to the cashier, and attempted to subtly fix it, as I didn't want her to come to harm, but she affected (possibly quite genuinely) not to understand the error. Took it to the assistant manager at the back of the store twice, who might have been confused, or might have been trying to resolve the situation without undue trauma to local staffing, but both times shut down, and the Google translate screen at the end said "...It is okay for you to leave."
So, reader, I did.
Will have to figure out how to subtly return it to the local folks, despite my own wing-and-a-prayer budgets on this leg of the trip. I once enriched the poor box at St. Pat's with a twenty when I was in a similar predicament stateside. I don't want anyone local to come to harm, but even though it's less than $25, this runs too close to American Privilege for my comfort.
At any rate, have decided that it won't drive me mad, and now I'm going to focus my mind on the work for the rest of the evening. Insh'allah, and pace the bakshish.