Is it permissible for an engaged couple to agree that the مقدم Mahr (dowry) will be eight million, of which only six million will be paid, and then inform the bride's family that the full amount has been paid? And what is the manner of repentance if that is not permissible?

The marriage is valid even if the Mahr (dowry) is not mentioned in it, for the Mahr is an exclusive right of the mature wife, and she has the right to accept less than the Mahr of her peers, and her guardian has no right to prevent her. If the wife agrees with her husband on a Mahr and is content with it while announcing another Mahr, that is permissible but disliked (Makrooh). If the wife is content with the secret Mahr, there is no blame on the husband, and even though demanding the public Mahr is obligatory, it is recommended for her to fulfill her promise to the husband regarding accepting the secret Mahr. If the marriage took place before the wife reached maturity, the agreed-upon Mahr with her guardian is established for her, and after reaching maturity, she is permitted to waive the Mahr or part of it.