Lumsum salary adjusted from march incentive earned

Recently company has deducted salary of last 12 months given in advance with the recently incentive earned and no where in the offer letter is written this condition only in sales incentive promotion skime it is written as a condition as salary will be adjusted from incentive..pls help to clearfy this condition.
Thanks

You have taken12 months' salary as 'advance' salary?

The entire amount has been deducted from the incentive?

Whereas there is no mention about how the salary advance can be deducted?

Then I would say that what you received as advance was not salary advance but incentive advance at the rate of 100% of annual salary recoverable from the annual incentive. Why the company has paid so much of amount as advance is not known to me but it was a very good move on the part of the company. But at the time of paying the same, they should have told you that the entire amount would be deducted in one lot from the actual incentive.

I don't think there is any illegality or anything against an employee.