The customers discovered the mistake Tuesday when 18-year-old Kennidi Rue opened the bag a few miles down the road to hand out tacos to her friends, Mlive.com reported Wednesday.
"This is all money in here," Rue remembered saying to her friends."It was the weirdest thing ever."
Rue was with her boyfriend, 20-year old Grant Kruse, and his friend, 25-year-old Luke Postma, at the time.
"We all kind of talked about it together and decided we should give it back. I'll never feel guilty about giving it back. I could never have that on my conscience," she said.
The friends called police for help and went with an officer to return the money.
Rue told Mlive.com that a manager burst into tears when they returned the sack filled with cash.
The store manager told police she had set the money bag down on the counter and another worker mistook it for the takeout bag for the next drive-through customer.