Walmart overpaid me and to remedy the error, I sent them a Cashier’s Check — check was rejected.

The cashier’s check was declined, and I was told to give them money. Employed still. Am I now legally obliged to pay them? Their response to me sounds sort of fishy-the store manager told me not to worry about it-I accept that they can only make it worse if Payroll gets involved (therefore the Cashier’s Check to return what I have not earned). I really don’t want to get fired for trying to be honest or go to prison.