I suspect you WILL have to pay it back, eventually... so why not put the difference into a savings account, so at least you get some interest on it whilst your employers (or their accountants/payroll dept) get their act together). I was overpaid for some six months a few years ago and the first I knew about what action they were taking was when they simply did not pay me at all one month... (I didn't realise I'd been overpaid, I thought it was a payrise doh !). As it was the December payroll and we get paid in the middle of the month, you can imagine how nice Christmas was that month !