Dear Vputhran,
In so far the deficiency of the value of the stamp paper is concerned the same can be cured subsequently before going for the registration and or initiating and prosecuting legal remedy.
The other question is about the evidentiary value of the SLA. For this reason only I have stated "In usual practice Rs.100/- Stamp Paper is used for this purpose. In so far Evidentiary value is concerned then subsequently the fact of procuring the stamp paper for a particular purpose (when any body goes to procure a stamp paper he has to sign the register and mention about 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 a enforceable contract and has a binding force. For registered and unregistered documents kindly see sections 17 and 49 of Indian Registration Act. I hope this satisfies your query. All the best God Bless