If the marriage contract was done without any mention of the mahr, the marriage contract is valid and the wife is entitled to a mahr like that of her peers.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (7/182): Marriage is valid without stating the dowry according to the opinion of most of the scholars. That is indicated by the verse in which Allah, may He be exalted, says (interpretation of the meaning): There is no sin on you, if you divorce women while yet you have not touched (had sexual relation with) them, nor appointed unto them their Mahr (bridal money given by the husband to his wife at the time of marriage) [al-Baqarah 2:236]. It was narrated that Ibn Masood was asked about a man who married a woman but did not state what her dowry would be (did not appoint unto her her mahr), and he did not consummate the marriage with her before he died. Ibn Masood said: She is entitled to a dowry like that of her peers, without exaggeration or falling short, she has to observe the iddah and she is entitled to inheritance. Maqil ibn Sinaan al-Ashjai stood up and said: The Messenger of Allah (blessings and peace of Allah be upon him) issued a verdict concerning Birwa bint Waashiq, one of our womenfolk, that was similar to your verdict.
Narrated by Abu Dawood and at-Tirmidhi, he said: A hasan saheeh hadeeth. End quote.
Marriage without stating the mahr is of two types. The first is that in which the mahr is not mentioned at all, and the man gives his daughter in marriage without any mahr, so he says: I give you my daughter in marriage, and the suitor says: I accept - without mentioning any mahr, as happened in your case.
The second type is that in which the mahr is mentioned without specifying its amount, such as if the suitor says to the wali: I will give you what you want of the mahr, or the wali says to the suitor: Give whatever you see fit, and so on.
The wife is to be given a mahr like that of her peers in both cases.
The mahr of her peers is to be defined by the qaadi (judge), so as to avoid conflict. But if they agree on something without referring to the qaadi, there is nothing wrong with that, because they have the right to do so.
It says in Zaad al-Mustaqni: It is valid to do the marriage contract without mentioning the mahr or mentioning it without specifying it. The wife is entitled to a mahr like that of her peers by virtue of the marriage contract, and the amount is to be specified by the judge. If they agree upon something before that, that is permissible. End quote.
Shaykh Ibn Uthaymeen (may Allah have mercy on him) said: The words If they agree upon something before that, that is permissible mean: if they agree on it without referring to the judge, they have the right to do that. In other words, there is nothing wrong with that. So if they say: Why should we go to the qaadi? Let us agree on something between ourselves, and the husband offers a mahr of one thousand, and she asks for two thousand, and some people intervene and suggest one thousand five hundred, and so on, there is nothing wrong with that, because they have the right to any agreement they reach.
End quote from ash-Sharh al-Mumti, 12/315
Thus you will see that the marriage contract is valid and that the mahr on which you have agreed is also valid, if the wife is mature and of sound mind.
And Allah knows best.
Reproduced from Islam QA