General Hr policy when the employee doesn't serve full notice period?

Hi all, I need some clarifications on the general HR policy for payment deduction for an organization if the notice period is not served. Let me give you a specific example: I am working in a start-up organisation. He has received an offer letter via email only, no hard copy is signed and not received a company appointment letter. As per the offer it’s mentioned 3 months’ notice period, nothing is mentioned about payment deduction if the notice period is not served fully and also nothing about the full and final settlement. Now A puts in his resignation on 1st May 2021 stating that he will be serving the notice period of a maximum of 1.5 months instead of 3 months. In this period he would be assisting in the smooth transition of his work and responsibilities to the person who takes over his role. On 10th May 2021, Employer informed that salary is under hold the of last month i.e. February’21 and will be released in the full and final statement. Now the Employer is saying that 2.5 months salary will be released in the full and final statement and there will be deduction “1.5 month full salary” as you have served only 1.5 month notice period instead of 3 months. My question here is – Is the company can hold the salary of Feb month i.e. before put in of resignation? and can they deduct any payment (as nothing is mention in the offer letter about deduction), if yes can they deduct a 1.5-month full salary as compensation for his notice period not served?? Please advise what should I do? Regards, Dhiraj