He said that his wife would be haraam to him if he took a second wife and now he wants to marry again
Question

                1 – What is the ruling on a man who follows the Maaliki madhhab if he said that his wife would be haraam to him if he took a second wife, meaning to close the door to a second marriage more than to divorce his wife if he took a second wife? 

2 – What is the ruling on a man who swears that he will divorce his wife if he commits zina (dultery), then he commits zina? 

3 – If the divorce (talaaq) is valid in either case, is there any solution to enable him to keep his wife? Please note that his wife does not live with him in the same city, and he does not want to divorce her, but he wants a way out that will allow him to marry another wife so that he will not fall into zina (adultery) again and commit sin and his wife become forbidden for him.
Praise be to Allah.
    Firstly: 

    Divorce has not taken place in this case, but he must offer 
    kafaarat yameen (expiation for breaking a vow) if he takes a second wife, 
    because by swearing that oath he meant to stop himself (from marrying 
    another woman), not to divorce his wife. 

    Ibn ‘Uthaymeen, Fataawa Manaar al-Islam, 2/584. 
The questioner’s words, 
    “meaning to close the door to a second marriage more than to divorce his 
    wife if he took a second wife” appear to mean divorce and stopping himself 
    at the same time, but the idea of stopping himself is stronger. The ruling 
    does not change because the emphasis is more on stopping himself. 

    Shaykh Khaalid al-Mushayqih  

    Secondly: 
If what he intended when 
    he swore the oath was divorce, then divorce has taken place. If he committed 
    that sin, the divorce has indeed taken place. But if what he intended was to 
    stop himself comitting this sin – which is usually the case when such words 
    are spoken – then he must offer kafaarat al-yameen. 

    Fataawa al-Talaaq by Shaykh Ibn 
    Baaz, 1/141 

    The Muslim should not play about with the word of divorce (talaaq), 
    and swear to divorce his wife as a means of preventing himself from doing 
    something, because many of the scholars think that divorce actually takes 
    place in such cases, even if he did not intend that it should take place. 

    Thirdly: 
Our advice to the 
    questioner is that he should repent sincerely from the zina to which he has 
    confessed, for this is one of the most repugnant of sins, which generates 
    darkness in the heart and on the face of the one who does it. He must strive 
    to follow the means that will prevent him from committing this sin. The 
    scholars (may Allaah have mercy on them) have stated that marriage is 
    obligatory if a man fears that he may fall into sin if he does not marry. 

    Al-Mughni, 9/341 

    So you must strive to bring your wife to live with you in the 
    same city, because this is a way of treating her in a kind manner. You 
    should realize that she is suffering just as you are. You should not be 
    concerned about protecting yourself from falling into sin, then leave your 
    wife to suffer, for that is contrary to the kind treatment enjoined by 
    Allaah and His Messenger. If you cannot manage to do that, then you should 
    take a second wife, but that does not mean that your first wife will be 
    divorced, as stated above; rather you will have to offer kafaarat yameen. 

    Kafaarat yameen is mentioned in the aayah (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”

    [al-Maa’idah 5:89]

    May Allaah send blessings and peace upon our Prophet 
    Muhammad, and upon all his family and companions.