Mahr is the woman's right; if she claims it, the husband will have to pay an amount that one of her peers would accept as Mahr. This is because the Mahr is a return for allowing the husband to have the pleasure of sexual intercourse with his wife.
If a woman willingly waives her right to the Mahr or part of it, (Part No. 19; Page No. 75) it will be permissible and does not affect the validity of the contract. This is because Allah (Exalted be He) says,
And give to the women (whom you marry) their Mahr (obligatory bridal-money given by the husband to his wife at the time of marriage) with a good heart; but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allâh has made it lawful).
May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions.