He vowed to marry a certain girl then he did not fulfil his vow
Question

                I vowed to Allaah that I would not marry any girl except this girl, and that if I married anyone else I would fast for three months. Then I found that I could not fulfil my vow, and I do not have any work so that I could feed the poor every day. What is the solution? May Allaah reward you with good.
Praise be to Allah.
      Firstly: 

    What the Muslim should do is avoid making vows, because the 
    Prophet  (peace and blessings of Allaah be upon him) told us not to do 
    that. 

    Al-Bukhaari (6608) and Muslim (1639) narrated that Ibn ‘Umar 
    (may Allaah be pleased with him) said: The Prophet  (peace and blessings 
    of Allaah be upon him) told us not to make vows and said: “They do not 
    change anything, rather they are a means of making the miser give 
    something.” 

    See also question no. 32724 and 
    36800. 

    Secondly: 

    The scholars (may Allaah have mercy on them) stated that vows 
    are of several types, such as a vow that is intended as an oath, which is 
    aimed at encouraging an action or avoiding it. 

    The ruling on such vows if they are broken is that the person 
    is given the choice between doing that action he vowed to do or offering 
    kafaarat yameen (expiation for breaking an oath). 

    Ibn Qudaamah said in al-Mughni (13/461): 
If a person makes a vow 
    in the sense of making an oath, to stop himself or someone else doing 
    something, or to make him do something – such as saying, “If I speak to 
    Zayd, then I must go on Hajj, or give my wealth in charity, or fast for a 
    year,” then this is an oath, and the ruling thereon is that he has the 
    choice between fulfilling what he swore to do, in which case he does not 
    have to do anything else, or breaking his oath, in which case he has the 
    choice of doing what he vowed to do or offering kafaarat yameen. This is 
    called the vow of argument and anger, and he does not have to fulfil it… 
    This is the view of ‘Umar, Ibn ‘Abbaas, Ibn ‘Umar, ‘Aa’ishah, Hafsah, and 
    Zaynab bint Abi Salamah. This is also the view of al-Shaafa’i. End quote. 

    It seems that your vow to fast for three months if you did 
    not marry this girl is of this type, because by doing that you intended to 
    encourage yourself to marry her. This is an oath. 

    See also question no. 45889. 

    Kafaarat yameen (expiation for breaking an oath) is mentioned 
    in the verse where Allaah says (interpretation of the meaning): 
“Allaah will not punish 
    you for what is unintentional in your oaths, but He will punish you for your 
    deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen 
    (poor persons), on a scale of the average of that with which you feed your 
    own families, or clothe them or manumit a slave. But whosoever cannot afford 
    (that), then he should fast for three days. That is the expiation for the 
    oaths when you have sworn. And protect your oaths (i.e. do not swear much). 
    Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, 
    lessons, signs, revelations, etc.) that you may be grateful”
    [al-Maa’idah 5:89] 

    See also question no. 45676. 

    To sum up: if you marry that girl, then you do not have to do 
    anything. If you do not marry her, then you have the choice between 
    fulfilling your vow of fasting for three months, or offering kafaarat 
    yameen. 

    And Allaah knows best.