How to change the end date of CoS?

[Marwan]

Hello
I applied for my visa extension for one year, but I received a letter from home office saying that there is discrepancy between my CoS end date and what I paid for IHS. They also sent me an email to pay IHS for two more years. I believe my employer has made a mistake and gave an end date for three years. The problem is that I can apply for Indefinite leave to remain in exactly one year so I won’t benifit from 3 year extension. And they can’t extend my visa for three years as this will mean a total of 7 years on Tier2!!
What can I do now? Can my employer change the end date of CoS?

Can you advise ASAP as they gave my only a week to pay extra £800 or to refuse the visa

Thanks