138 ni act & contract act

The signature as a witness makes no difference and can not make the agreement invalid when the facts of such agreement can be established.  Yes.  It can be challenged but the invalidity depends on other facts decided by the court., as there was no objection while his signature is obtained or afterwards as many such beneficiaries may be illiterate or innocent and might have signed in the usual course.

How NI 138 is relevant in the context of the limited facts posted?