If a person enters dar al-harb with a promise of safety, it is not permissible for him to betray them with regard to their lives or their wealth
Question

                In some stage of my life when I was rash careless young man, I took 30000 Belgian frank from two banks. Then I returned to my country; with both accounts closing. What shall I do to get rid of this sin? I can not go back to Belgium. Is it enough to repent? Knowing that it is only one time I committed this sin.  Will I be called to account in front of the people of the banks at the Day of Judgment although they deal by interest? Shall I return this money to the banks? Knowing that I have a bit more than this amount for my marriage.
Praise be to Allah.
    What you did by taking the money mentioned from the two 
    banks, is a haraam action, because it was consuming people’s wealth 
    unlawfully, and Allaah says (interpretation of the meaning): 

    “O you who believe! Eat not up your property among 
    yourselves unjustly except it be a trade amongst you, by mutual consent”

    [al-Nisa’ 4:29]. 

    Even if the owners of the bank are kuffaar, this does not 
    make it permissible to take their wealth, because if the Muslim enters a 
    kaafir land with a promise of safety, it is not permissible for him to 
    betray them, and it is not permissible for him to harm them or take their 
    wealth. 

    Al-Shaafa’i (may Allaah have mercy on him) said in al-Umm 
    (4/263): If some Muslim people enter a kaafir land under a contract (of 
    peace and safety), then the enemy should be safe from their harm until they 
    leave, or until the contract expires, and the Muslims have no right to wrong 
    them or betray them. End quote. 

    Al-Sarkhasi said in Sharh al-Siyar al-Kabeer (2/507): 
    If a group of Muslims comes to a kaafir land and says, “We are the envoys of 
    the caliph” and they produce a letter like that of the caliph, or do not do 
    that, and that is a trick on their part to deceive the mushrikeen, and they 
    allow them to enter, so they enter the kaafir land, it is not permissible 
    for them to kill any of the kaafirs or to take any of their wealth, so long 
    as they are in their land, because if what they said or showed to the 
    kuffaar were true, then they would be safe and secure from the kuffaar and 
    the kuffaar would be safe and secure from them, and is not permissible for 
    them to wrong them in any way. 

    If they asked them for safety and they granted that to them, 
    then they are obliged to be honest with them. 

    The same applies if they say: We have come with the intention 
    of trade, but they aim is to kill some of them, because if they were really 
    merchants as they claimed, it would not be permissible for them to betray 
    the kuffaar. End quote. 

    It says in al-Hidaayah – which is published with 
    Nasab al-Raayah (4/304): If a Muslim enters a kaafir land as a merchant, 
    it is not permissible for him to transgress against anything of their wealth 
    or their blood, because he has guaranteed not to transgress against them by 
    asking them for safety, and transgressing against them after that is a 
    treachery, and treachery is haraam unless the king of the kuffaar betrayed 
    them and took their wealth or imprisoned them, or someone else did that with 
    the knowledge of the king and the king did not stop him, because they are 
    the ones who broke the treaty. This is different from the case of the Muslim 
    prisoner, because he is not under their protection, so it is permissible for 
    him to transgress against them even if they released him willingly. End 
    quote  

    See also question no. 
    72384 and the fatwa of the Standing Committee quoted therein. 

    Based on the above, you have to return the money to its 
    owners and not take the matter lightly, because one of the conditions of 
    repentance is restoring the rights of those who have been wronged. 

    We ask Allaah to guide us and you. 

    And Allaah knows best.