The mahr (ṣadāq) is an obligatory gift the husband gives to the wife herself as a right of the marriage contract, and Allah commanded, "And give the women their dowries graciously." It is her sole property to keep, spend, or invest as she wishes; it is not owned by her father or guardian, and no one may take it from her without her free consent. Allah added, "But if they give up willingly to you anything of it, then take it in satisfaction" — showing it is entirely hers to voluntarily forgo. It may be large or small by mutual agreement, and its purpose is to honour the woman, not to "purchase" her.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.