What happens if an employee refuses to extend his probation period?

Dear sir/madam,

I signed an offer letter with my employer which clearly stated my probation period to be 90 days.

Yesterday, about two weeks after my 90-day probation period was completed, I was asked by my manager if I would accept extending my probation by another 90 days.

Is this legal?

And if I refuse and get terminated, what compensation will I be entitled to?