Bahir Khan

I am on a tier-4 student visa and I am part-time worker (working 20 hours a week). This month, my employer made a mistake and exceed my working hours. I worked 80 hours but got paid more than that and my pay slip shows 119.9 hours. After contacting my employer, they said its a human mistake by one of our manager while calculating you working hours, However you will be paid less in the next month. In order to be on the right book what should I do? How should I tell the HM team that I did not exceed my working hours and adhere the immigration conditions.