Bismillah

Al-jawab billahi at-taufeeq (the answer with Allah's guidance)


Regarding the above situation, as long as both contractors and witnesses know the amount of Mahr that was agreed at the time of the contract of Nikah, then the former amount will presume as the Mahr and not the latter amount. This will not effect the validity of Nikah.

In addition, it is mentioned in Hidaya, that the contract of Nikah is valid even if no Mahr is named in it (That is, it is not like other commutative contracts where a counter value has to be paid from both sides). The reason is that Nikah is a contract of joining and union in its literal meaning and is, therefore, complete with two spouses. Thereafter, Mahr is obligatory according to Shari'ah. Consequently, there is no need of mentioning it for the validity of the Nikah.

(Hidaya, Chapter: Mahr (Dower))


And Only Allah Ta'ala Knows Best.