Related How do you refer to a 50 Euro bill in German?.
American English has an inventory of slang terms for amounts of money, not just the the notes themselves, which seem to be covered by the other question. For example 100 grand = 100 large = $100,000, but not in $1000 bills. A "big one" can mean $1, $100, or $1000 depending on context, and $1 has many nicknames, "buck", "smacker", "clam" etc. Again, these refer to the amount, so I'd say 20 bucks for 20 singles, 4 fives or 1 twenty.
I just saw in the subtitles for a show set in the US 90 Riesen for $90,000, which I gather translates literally as 90 giants, so I wondered if there are similar slang terms in German. For example would 90 Riesen be "€90,000" (= 90.000 € to German speakers) in Germany and Austria? (Switzerland is not in the eurozone, so I imagine they might have different words.) There is a Wikipedia article Slang terms for money but it doesn't really cover German speaking countries, and only mentions Teuro for Euro. There doesn't seem to be a similar article on German Wikipedia.