Dear Sagnik,
In some cases, the courts or authorities may give sanction to admit an unstamped or insufficiantly stamped document on payment of penalty in addition to the stamp duty fixed to be payable under the law.
However, demand Promissory notes and a usance Bill of Exchange will be considered invalid if they are not stamped or insufficiantly stamped.
The stamp inspecting government officer may impound your unstamped or understamped document
So, to avoid any impoundment of such document, you should take care that they have been properly/adequately stamped.
Further, you should take extra care before submitting documents before a court/Tribunal or Government bodies.
Regards,
Veeral Gandhi