Mistakenly overpaid by Employer, what to do?

Hi,
I am under a 1 year contract in an agency, designated to work for a client which is a big IT company. For the past 8 months, I started my standby hours (on call) shifts thus my employer started to pay me for the standby hours that I rendered as well. The standby hours pay is somewhat equal to my hourly rate by the way.

Now here is my dilemma, Just last month my hr representative mentioned that there was an error with the standby hours pay that they are actually giving me. Turned out I was overpaid; the stand by hours is apparently not equal to my hourly rate but just 20% of the rate per hour.

This is very concerning for me, because they also mentioned that they will deduct the over payments to me eventually which is a very huge amount.

I really need an advice as what to do with this? My contract with them will expire in 3 months and with the amount that they will deduct to me, it's impossible for me to pay them that soon.

Thank you!

was it stipulated in the contract that the standby rate is 20% of the regular rate? if so, then you cannot dispute the deduction which is to come. It is legal.

No, there is nothing stated whatsoever with regards to the rate of the standby hours. That's why I assume that in the 8 months (which had consistent rate) had the rightful amount that they are giving me. I didn't suspect at all that there was some discrepancy.

Whether stated in your contract or not, the question is 'Was overpayment really happened'? If so, you are obligated to return the same. Base on equity and 'fair play', any amount not due an employee, if given by mistake, the latter has the duty to return the same.