As rightly said, we need to see the relevant clause in the Offer/Appointment letter. The Retention Bonus in this concern is a future payment, payable only after the completion of three years' service. The deduction of any amount towards payment not yet made is badly conceived and perhaps goes contrary to the very purpose. What would have been appropriate is to put a condition that if the concerned employee leaves before completing three years of service, he will not be entitled to receive the Retention Bonus.
Regards, Vinayak Nagarkar
HR Consultant