The stamp duty payable on certain documents in neighboring states is less. So whether the documents executed outside Karnataka but relate to property in Karnataka are valid?

Any document has to be stamped as per the rates fixed by the State where it is operative but there is no prohibition on execution of such documents outside the state. However, stamp duty on such documents depends on the rates fixed by the state where it is operative. If the stamp duty payable on such documents is more in Karnataka, the difference amount has to be paid i.e., less the stamp duty already paid in other state.  This should be done within one month after it was brought into Karnataka.