Company not allowing early release despite payment

My company is not allowing an early release despite me providing them payment option for the left days. I want to leave 30(out of 90) days early . They r demanding that i serve full NP otherwise they will hold my experience letter. I need the experience letter as i had worked there for 3years. Can anybody asist on how to leave this company with experience letter? --------- Termination clause a) By the Employee: The Employee may terminate the employment at his / her discretion by giving 90 (Ninety) days prior notice or payment in lieu of notice. b) By the Employer: The Employer may terminate your employment at its discretion by giving 90 (Ninety) days prior notice. However, if any disciplinary misconduct or any other severe action/ behavior is noticed, the Employer may terminate your employment with immediate effect or shortened notice period. The Employer may at its sole discretion, relieve you of your duties anytime during the notice Period and in that event, you will be paid salary up to the last full working day only. If you are Working on any assignment/s during the notice period, the Employer may require you to complete such assignment/s to the satisfaction of the Employer, before relieving you from your services.