Dear Vputhran,
Insofar as the deficiency of the value of the stamp paper is concerned, the same can be cured subsequently before going for the registration, or initiating and prosecuting legal remedy.
The other question is about the evidentiary value of the SLA. For this reason, I have stated, "In usual practice, Rs. 100/- Stamp Paper is used for this purpose. Insofar as evidentiary value is concerned, then subsequently the fact of procuring the stamp paper for a particular purpose (when anybody goes to procure a stamp paper, he has to sign the register and mention the purpose for which the stamp paper is required). Then the same has to be witnessed and notarized or affirmed by the Oath Commissioner, which makes it binding on the signatory parties."
For all practical purposes, a completed, signed, executed, and witnessed SLA is an enforceable contract and has a binding force. For registered and unregistered documents, kindly see sections 17 and 49 of the Indian Registration Act. I hope this satisfies your query. All the best and God Bless.