is repayment of joining bonus by an exiting employee legally enforceable ?

I signed an employment contract that paid me joining bonus every six months with the conditions that if I leave within 2 years, I have to return all the bonus received from the employer. After joining, I didn't like the job and decided to quit after 13 months. Now I am being asked to pay the joining bonus including the tax. The problem is 2 such bonus transactions took place in the financial year 2019-2020 and form-16 included the bonus as my taxable slab.

now,
how do I get my tax back in this case ? are such contracts even legally valid as this bonus was paid from a retention perspective.