What the Muslim must do is be a good example of trustworthiness, fulfilling covwnants and good attitude. The fact that Muslims had these qualities was a cause of many kuffaar entering Islam when they saw the beauties of Islam and the good attitude and conduct of its people.
Treachery and betrayal are forbidden in Islam, whether that is with a Muslim or a kaafir. What the Muslim is required to do is to respect the covenant between him and the country where he is or to which he has travelled, even if it is a kaafir country. The fact that it is a kaafir country does not make it permissible to betray it and consume its wealth unlawfully.
So long as that country has granted safety to the Muslim who entered it, and it is giving him protection and protecting his wealth, then he is required to be honest towards that country, and it is not permissible for him to betray it or cheat it.
Based on that, that country is within its rights to admit whomever it wants and refuse whomever it wants. If any Muslim enters that land, it is not permissible for him to deceive them or cheat them, whether that is done to the state itself or to its citizens, institutions or companies. He does not have the right to insist that they admit his family into their country or grant him permanent residency.
Ibn Qudaamah (may Allah have mercy on him) said:
As for betraying them, it is haraam, because they only granted him safety on condition that he refrain from betraying them and that he protect them from himself (i.e., harm caused by him). Even if that is not mentioned in so many words, it is implied and understood. End quote.
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
It is not permissible to take the wealth of a kaafir, when they have trusted one, by dishonest means, because people may travel together on the basis of trust, and trusts must be rendered back to their people whether they are Muslims or kaafirs. End quote.
Based on that, what you have to do is return to them the money that you took from them.
And Allah knows best.
Reproduced from Islam QA