US authorities have decided to return a $US12,000 tip to a Minnesota waitress that police believed was drug money.

Stacy Knutson, of Moorhead, says a customer told her she could keep a takeaway container she left behind at the Fryn' Pan restaurant.

The box turned out to contain $US12,000 ($A11,700) in bills in various denominations.

Police initially told Knutson she could keep the money if no one claimed it, but later said it was part of a drug investigation.

But after the case drew national attention, Assistant Clay County Attorney Michelle Lawson said the money could not be tied to a criminal investigation, and that Knutson would get a cheque.

Knutson said she believed the money was an anonymous gift from someone who knew her family had severe financial difficulties.

AP