Sales/Service personnel
I have come across two different methods of Retention allowance:
1. Deduct a certain percentage of the salary every month and pay back when the employee resigns. This amount is generally used to adjust any payment due from an employee. Strictly, this cannot be called Retention Money, and the deduction is made on the fixed salary. Such a deduction is illegal, and no employer can refuse to pay back this money. However, some smaller companies resort to such practices, mostly to manage Sales and Service personnel who disappear without any notice.
2. Retention allowance specified as payable on completion of a stipulated time period. This is the real Retention allowance and is payable, as I stated earlier, only on fulfilling the contractual obligation category.
You seem to be coming under category 1 where the amount is deducted from your salary and retained with the employer as retention money to be adjusted against dues payable by you. In my opinion, if you do not owe any sum, the Company must pay this back to you.