I think that you can negotiate repayment with them, and just say the worse thing that can happen, you don't get the job, then I think they can freeze this debt, until your in a more financial state to pay it back ... At the end of the day, if you owe this money back to them, then you will (sorry) have to pay them back..
Also I could be wrong here, but if they have over paid you then your tax would of been higher, and maybe if that is the case you were paying more on tax because of the higher wages, you maybe able to get some of your tax back, and it be great if it comes to the amount they say you owe them. Just a thought 
Good Luck Steve
Cheers
Roo
