Is it permissible for a girl and her fiancé to secretly agree upon a mahr (dowry) different from what is written in the marriage contract, knowing that the parents insist on a specific mahr that the fiancé deems excessive?

If a woman is of sound mind and the suitor is suitable, she may consent to marry him for less than the customary dowry (Mahr al-Mithl). Her guardian has no right to prevent her; if he does, he is considered to be unjustly preventing her marriage (adil), and the ruler (judge) shall marry her off. It is permissible for the man and woman to agree upon any dowry, whether small or large. It is permissible to declare a dowry that is greater than the amount secretly agreed upon, and she is only entitled to claim the secret dowry. After the marriage contract (aqd), the woman has the right to waive a portion of the dowry.