She vowed to marry off her daughter for a large dowry and now she wants to reduce it
Making it easy to get married, and not going to extremes in the mahr and in the wedding expenses, are praiseworthy things, and they are good and bring blessing to the couple. This is merciful towards the husband and spares him from becoming burdened with debt, and it is merciful towards all young men and saves them from temptation and makes it easy for them to remain chaste. So reducing the amount of mahr is a good deed and we hope that you will be blessed for doing that, by Allaah’s leave.
Shaykh Ibn Baaz (may Allaah have mercy on him) said: Going to extremes in women’s dowries is makrooh. It is Sunnah to ask for little and to be easy-going with regard to that. End quote from Fataawa al-Shaykh Ibn Baaz (21/87).
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: What is your opinion about going to extremes with dowries and extravagance in wedding parties and especially in preparing for the so-called honeymoon? Does Islam approve of that?
He replied: Extravagance with regard to dowries and parties is contrary to sharee’ah. The most blessed wedding is the one that costs the least. The less the expense, the greater the blessing. The same applies to extravagance in parties which is forbidden in Islam and is included in the verse in which Allaah says (interpretation of the meaning):
“and waste not by extravagance. Verily, He likes not Al‑Musrifoon (those who waste by extravagance)”
Such things should be done in the manner prescribed in sharee’ah, and no one should transgress the limits or be extravagant, because Allaah forbids extravagance and says: “Verily, He likes not Al‑Musrifoon (those who waste by extravagance)”.
What may be said about the honeymoon is that it is worse and more hateful, because it is an imitation of non-Muslims, and it wastes a lot of money. It also leads to neglect of many religious duties, especially if this time is spent in a non-Muslim country, because they come back with habits and customs that harm them and their society. These are things which we fear will adversely affect the ummah. But if a man travels with his wife for ‘umrah or to visit Madeenah, there is nothing wrong with that, in sha Allaah.
End quote from Fataawa Islamiyyah (3/175).
In the event that the vow is not fulfilled – which is better for you, as explained above – then your mother must offer expiation for breaking an oath (kafaarat yameen) because a permissible oath does not have to be fulfilled, rather one has the choice between fulfilling it or offering expiation (kafaarat yameen), by freeing a slave, feeding ten poor people with the average kind of food that you eat, or clothing them. If none of that is possible then she should fast for three days.
Ibn Qudaamah (may Allaah have mercy on him) said: A permissible vow includes such things as wearing a certain garment or riding an animal… in this case the one who made the vow has the choice of doing it and thus fulfilling the vow or if he wishes he may forgo that and offer expiation (kafaarat yameen). End quote from al-Mughni (10/70).
May Allaah help us and you to do that which He loves and which pleases Him.
And Allaah knows best.