Is it permissible for the husband to keep the engagement ring after separating from his Christian wife without divorce? What is the ruling concerning this dowry if it was purchased with usurious money? What should he do if it is not permissible?

First: It is permissible to use a Visa card if it is free of Sharia (definition) prohibitions. However, a "credit card" is a credit card guaranteed by interest-bearing savings deposits, so it is not permissible to use it.

Second: If a purchase was made with a prohibited card, you must repent and seek forgiveness. It is not necessary to sell the ring or donate its entire price. Instead, you must donate the money acquired through usury, if known, to charitable causes as a way of getting rid of it and repenting. A marriage for which the mahr (dowry) was paid with unlawful money is valid, and the person concerned must repent and return the money to its owners if possible, otherwise, donate it. Unlawful money from usury is only the increase. If lawful and unlawful money are mixed, the unlawful amount is extracted, and the rest remains lawful. The problem lies in the original prohibited transaction, and you must extract the interest that accrued to you from it.

Third: You are not entitled to take anything from your wife's money, including the ring, except with her full consent. If the ring was her mahr or part of it, it is her right, and it is not permissible for you to reclaim it from her unless she voluntarily relinquishes it.