He gave his wife her mahr: a ring that he bought with riba

Praise be to Allah.

Firstly: 

There is nothing wrong with using a credit card so long as it is free of things that are prohibited according to sharee‘ah. However if it does involve any of these things that are prohibited, it is not permissible to use it. For more information, please see the answers to questions no. 97846, 102055 and 129976. 

The credit card is a card that involves riba (interest); as that is the case, it is not permissible to use it. 

Secondly: 

As the purchase has been made and it is not possible to undo it, what you must do is repent, ask for forgiveness and not go back to dealing in such haraam transactions again. You stated in your question that you have repented from that and have resolved not to go back to it; praise be to Allah for this guidance and we ask Allah to accept your repentance from you. 

You do not have to sell the ring and buy another one, and you do not have to give the entire price in charity. As well as repenting and asking for forgiveness, all you have to do is give in charity any wealth that you have acquired by means of riba when you were aware of the ruling, giving it to any charitable cause by way of ridding yourself of it and repenting to Allah. 

Please see the answer to questions no. 1391 and 78289

Shaykh Ibn Baaz (may Allah have mercy on him) was asked about someone who married using both halaal and haraam wealth; after a few years, his conscience woke up and he realised that he had got married using haraam wealth; what is the ruling on his case and what must he do? 

He replied: the marriage is valid and is not affected by that, so long as he fulfilled the conditions of marriage by marrying her with her consent, in the presence of her legitimate guardian (wali) and in the presence of two witnesses, at a time when there was no impediment to marriage. The fact that the mahr came from haraam wealth does not affect the marriage, if the conditions of marriage were met. Rather there was only a problem with some of the wealth that was acquired by haraam means. This does not affect the marriage, but he has to repent to Allah, may He be glorified and exalted, for what he took by haraam means, and he has to return the money to its rightful owners if he stole it or misappropriated it from someone. If it is not possible to do that, then he should give it in charity on behalf of its rightful owners, donating it to charitable causes such as the poor and needy, repairing roads and washrooms around the mosques, and so on. But as for the marriage, it is valid. End quote from Fataawa Noor ‘ala ad-Darb (3/1578)

http://www.binbaz.org.sa/mat/19656/print

You should understand that what is prohibited in the case of riba is the interest earned, not the original capital which was halaal. Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

With regard to the one who deals in riba but most of his wealth is halaal, that is because if he sells one thousand for one thousand and two hundred, it is only the extra amount that is haraam. If his wealth is a mixture of halaal and haraam, the halaal portion does not become haraam because of this mixing; rather he may keep the portion that is halaal. Similarly, if the wealth belongs to two partners, and the wealth of one is mixed with the other, it may be divided between the two partners. Similarly, if a person’s wealth is a mixture of halaal and haraam, the haraam portion should be estimated and removed, and the rest is permissible for him.

End quote from Majmoo‘ al-Fataawa (29/273) 

In that case, the problem is not the entire price of the ring; rather it has to do with the transaction which was haraam, and you have repented from that. The benefit that you got from this haraam transaction is what you should work out and pay it from your wealth. 

Thirdly:

You have no right to take anything from your wife’s wealth, neither the ring nor anything else, unless she gives it willingly. What appears to be the case is that she gave you the ring based on what happened between you, or because she thinks that the fact that you have separated means that she should return the ring to you. But this is not correct, because her mahr is due from you regardless of whether separation has occurred between you or not. So long as the ring is her mahr, or part of her mahr, she is entitled to keep it regardless of whether you get divorced or not, and it is not permissible for you to take it back from her unless she gives it up willingly. 

Please see also the answer to question no. 101758.

And Allah knows best.