The marriage contract was done without any mention of the mahr (dowry).

When the marriage contract was done, all the conditions were met of
having two witnesses and the wali (guardian), but when the marriage
contract was completed, the following happened:My hand was put in the
hand of the bride's guardian, but he read the wording of the marriage
contract from a piece of paper, namely"Zawwajtuka… (I give to you in
marriage…)", And he mentioned her name but he did not mention the
mahr. I replied to him with the word "Qabiltu (I accept)", but after
that I agreed with the bride on a very smallamount of money. Is what
happened valid and is the marriage contract valid or not?
Praise be to Allah.
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 validwithout stating the dowry according to the opinion of
most of the scholars. That is indicated by the verse inwhich 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
moneygiven by the husband to his wife at the time of marriage)"
[al-Baqarah 2:236]. It was narrated that Ibn Mas'ood 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 Mas'ood said: She is entitled to
a dowry like that of her peers, without exaggeration orfalling short,
she has to observe the 'iddah and she is entitled to inheritance.
Ma'qil ibn Sinaan al-Ashja'i 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 statingthe 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,
ashappened 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 theyagree 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 todo the marriage
contractwithout 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. Ifthey 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 itwithout 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.