Can a company force me to do a buy out if it is not mentioned in the offer letter?

I have resigned. As per offer letter, I have to serve notice 90 day notice period, but I am planning to only do 30 days notice. Company is telling me I have to do work for 45 days without salary and this will be considered as buyout. When I asked employer why this is not mentioned in offer letter, he said they usually don't include this in the offer letter. Is there anything else that I should be careful of and what should I do in this situation if I am sure that I do not want to serve 90 days? As per my offer letter, this is the only clause given if notice period is breached: 6.3. If you decide to resign from their position before completing the full threemonth notice period, the following conditions will apply: (a) The salary for the notice period will be withheld, along with any arrears due to you. (b) Any additional accrued benefits such as annual bonuses, may also be forfeited at the discretion of the company. (c) The company reserves the right to recover any financial losses or damages incurred due to your non-compliance with the notice period. (d) You shall not be entitled for any Relieving or Experience letter.