“each of us” vs “both of us”

A guy said that he will give them 20 bucks if they wash his car. So they asked him that question to make it clear. But I am confused by their reply:

Is that 20 bucks for each of us or both of us?

The guy said "Both."

Then how much are they going to get? Total 20 bucks or 40 bucks?

"Each" is a determiner and is used for single things: "Each of you need to make a plan for next week's meeting." or "Each employee will receive a bonus based on their performance."

"Both" is also a determiner but is used for things considered together as in a group, whole or more commonly, two items or people. "Both Jack and Jill work in accounting on Friday." or "Both of my sisters live in New York."

"Both" and "Each" can be followed by "of" as in the above examples

In your sentence, "Is that $20 for each of us or both of us?" what the speaker wants to know is if he or she will have to share twenty dollars with the other person (each will receive $10) or if they will each get $20. On paper, the use of the word "that" is confusing because it is either talking about "that" $20 bill or "that" as in an agreement that they have. During the conversation, it would have been clear because of the context and also follow up: "That's 20 for each of you." or "That's 20 for both of you to split."