To exchange money with someone whether it be a cashier, friend, or lonely drifter in order to cheat them out of money by asking for change that is greater than the amount due.
Ben- Hey I just gave that guy a twenty for my share of the pizza and he only gave me six bucks!
Brett- Dude, you just got fastcashed!
You give a cashier a five and say, "Out of ten" sometimes they will not catch it and give you change for a ten. But watch your ass because sometimes you are the one getting fastcashed.
Paying unjust fees like taxes or feeling like you are being overcharged for something. If you feel uneasy about paying a fee, you can say you are being fastcashed.