He vowed that he would not get married until his mother memorized a portion of Qur’aan, but it is very difficult for her
Question

                I have vowed that I will not marry until my mother memorises 3 ajza’ of the Quraan. This made her very upset and she did not memorise except three chapters (an-naba’, an-nazia’at, and Abas) and it is difficult for her. Am I considered a bad son as I upset her? Am I allowed to marry before she completes the 3 ajza’? What shall I do?.
Praise be to Allah.It is not permissible for 
    anyone to oblige his mother to do something that is not obligatory in 
    sharee’ah and causes difficulty for her. So long as your mother is not able 
    to memorize except with great difficulty, then you have no right to oblige 
    her to do that. You have to apologize to her and try to please her, and 
    offer kafaarat yameen (expiation for breaking an oath), because the vow that 
    is aimed at encouraging a person to do a specific action comes under the 
    same ruling as an oath (yameen). If you offer the expiation for breaking an 
    oath, then the vow is no longer binding, and your mother does not have to 
    memorize anything that she cannot. 
Ibn Qudaamah (may Allaah 
    have mercy on him) said, discussing different types of vows: The vow that is 
    made in the form of an oath to encourage a person to do something or to stop 
    them doing something, when the intention is not to make a vow or do an act 
    of worship, comes under the same ruling as an oath (yameen). End quote from
    al-Mughni (10/67). 
You have to please your 
    mother and offer the expiation for breaking an oath, and get married. 
For detail on the expiation 
    for breaking a vow, please see the answer to question no.
    45676. 
And Allaah knows best.