Dear friend,
When you joined the company, it must have issued you the appointment letter. In the clause about separation, please check whether any conditions are mentioned about the notice period and acceptance of pay in lieu of that. Many companies, though they accept payment in lieu of the notice period, also introduce a clause stating that the administration reserves the right to do so.
Companies include a clause of the notice period in the appointment letter because the sudden exit of an employee could bring discontinuity or even disruptions in the operations. The notice period is a transit period or buffer period during which the company can arrange the replacement of the exiting employee. Now, if the employees show willingness to pay in lieu of the notice period, then the very rationale behind the notice period could be defeated. Hence, your company could be refusing to accept payment in lieu of the notice period.
Thanks,
Dinesh Divekar