How to claim retention bonus tax refund?

I was given a retention bonus lets say Rs X (post Tax received approx 0.7x) in Dec 2015 (FY 2015-16) with the condition that if I leave the organization during the next two yeras I would need to return the amount. i'm moving on from my organization in March 2017 and I need to now return the pre tax amount to the company. The organization is saying that since this is in the next FY they will not be able to adjust the tax deducted on the retention bonus amount in the Form 16 for current FY. My questions related to how can I claim the tax refund are: 1. Should I submit a revised ITR for FY 2015-16 now in March 2017 reducing the income already submitted and claim refund in that OR 2. Should it be done in the next ITR (FY 2016-17) that I will be filing this year. If so, under which heads should this amount be reduced? In whichever option (from above mentioned or any other way), what letter shall I take from my current company? Also, is it better if the company deducts the bonus to be paid back from my final dues or I pay them a separate cheque for the amount? Since the amount is significant for me, is it better to go through a CA's office for claiming this refund or one can do this online easily Looking forward to your much appreciated advice.