She vowed to marry off her daughter for a large dowry and now she wants to reduce it
Question

                My mother vowed that if she gave birth to a daughter, she would do a seven-day wedding for her and the mahr would be 7000 Omani riyals. Now the problem of high dowries is widespread and that has led to a lot of women remaining unmarried. What is the ruling if we do not fulfil this vow and we make it easy for the girl to get married?.
Praise be to Allah.Firstly: 
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)”

    [al-An’aam 6:141]
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). 
See also 
    questions no. 10525 
    and 12572. 
Secondly: 
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.