Her husband gave her a mahr of a car that he bought by means of riba; now he wants to take it back so he can buy another car by permissible means

So long as your husband gave you this car as a mahr and you
took possession of it, it has become your property, so it is not
permissible for him to take it back from you against your wishes. Allah,
may He be glorified, has forbidden husbands to take anything of their
wives’ mahrs except that which the wives give willingly. Allah, may He be
exalted, says (interpretation of the meaning):

“And give to the women (whom you
marry) their Mahr (obligatory bridal money given by the husband to his wife
at the time of marriage) with a good heart, but if they, of their own good
pleasure, remit any part of it to you, take it, and enjoy it without fear of
any harm (as Allah has made it lawful)”

[an-Nisa’ 4:4]

“But if you intend to replace a
wife by another and you have given one of them a Cantar (of gold i.e. a
great amount) as Mahr, take not the least bit of it back; would you take it
wrongfully without a right and (with) a manifest sin?”

[an-Nisa’ 4:20].

But… If your husband wants to get out of this riba-based
transaction by selling this car and paying off the remaining riba
instalments, then buying another car for you by permissible means, then our
advice to you is to help him to do that, and seek reward for that from
Allah, may He be glorified. Let your intention be to help him in repenting
and to get out of this prohibited transaction, for Allah has decreed
kindness in all things, and the most deserving of people of your kindness is
your husband.

 And Allah knows best.