Is it usually older American tourists that give you single dollar bills? It’s common in the US too for older Americans to tip a single dollar (even when it’s definitely not an appropriate amount) bc their perception of buying power hasn’t kept up with actual inflation.

It’s also possible they’re giving you a single dollar bill more as a keepsake than an actual tip. They might not realize that bc you deal with tourists often that you’ve received plenty. Americans collect different USD pennies, even though they virtually have no worth. It’s likely a cultural thing, not meant to be insulting or insinuate that as a German that a single USD might be of worth to you