Technically, "here are" is correct, because "dollars" is plural, but I don't think I've ever heard anyone say "here are twenty dollars." At least in the US, pretty much everybody says "here's twenty" or "here's twenty dollars." I'm not sure why we think of a sum of money as always being singular, but that's how we say it. (Edited to add: We would also say "Twenty dollars is a lot of money." We would not say "are." If you use the plural "are," then it sounds like you're talking about twenty individual dollar bills. We use the singular "is" in order to show that "twenty dollars" is a sum of money, not a collection of individual dollars.)

Unless talking about individual coins or bills, an amount of money such as $20 is singular. More information here: https://erinwrightwriting.com/use-plural-singular-verbs-money/

"# dollars" is considered singular. ie. "Here is 20 dollars", "Here's 20 dollars" or "Here is 10 dollars", "Here's 10 dollars". So when the abbreviate the above, it remains singular. "Here's 20 dollars" = "Here's 20"