Dear Priyanshi,
Your company wishes to pay some amount of money to the employee. In lieu of the disbursement of this amount, which you are referring to as a "retention bonus," the employee should serve 'X' number of years in the company. If he fails to serve, then you wish to recover some amount from his FnF.
Suggestion 1: Stay Bonus
Rather than treading on the boggy terrain, I have two suggestions. First, instead of paying the "retention bonus," you may pay him a "stay bonus." If the employee serves for 'X' number of years and also delivers a desired level of performance, commit to the payment of a certain amount. He becomes eligible to get this stay bonus provided he does not quit the company.
Suggestion 2: Salary Increase
The second option is to increase the employee's monthly salary. Nevertheless, there will be no commitment to serve, nor will there be a recovery.
Caveat: Culture and Motivation
However, I would like to add a caveat. Whatever option you choose, there is a risk of making the company pecuniary-centric. Employees should stay in the company because they feel that there is a culture of autonomy or empowerment in your company or they feel that their self-actualization need is fulfilled by staying in the company. If you start paying a bonus for their stay, it is nothing but a trade-off. Please ponder over the unhealthy consequence of the culture of quid pro quo.
Thanks,
Dinesh Divekar