Service Certificate Only If I Give A Replacement?

Hi All,
I have a query relating to my resignation.
I work for a Manpower Consultant that deputes 500+ employees to various Organizations. In the firm only three of us are working on permanent basis.
As per my appointment terms, I am suppose to serve one month notice period. But I have got an offer to join within 15 days.
Now my present employer is saying that he will give the Relieving letter and Service Certificate only if I give a replacement and complete all the pending works.
Is it agreeable? What should I do in this case to solve the issue diplomatically?
Leads appreciated.

Dear Supriya,
Since your present employer has kept following two conditions:-
1. to provide a replacement and
2. to complete the pending work.
Since you work for a Manpower Consultant which deputes employees to various Organizations, I believe you must have developed contacts with similar organizations, you can take their help to find out a suitable replacement.
I am sure 15 days should be sufficient time to finished your pending work.
Failing to find out any suitable replacement, you can request your present employer to relieve you assuring him that you would provide your services whenever required or they have any problem.
BS Kalsi,
Member since August, 2011