I would like to know if you are allowed to use the wedding ring as mehr?
Mehr is the amount or item agreed upon at the time of nikaah which the husband has to give the wife. If the nikaah was conducted with the parties agreeing to the mehr being a ring, then giving a ring will be incumbent. If a sum of money or certain item was agreed upon, the husband does not have the right to arbitrarily change the agreed mehr and give something else. It is not permissible for him to say after the mehr was finalized that the wedding ring he is giving or gave is in lieu of the mehr, whereas the mehr agreed upon at the time of nikaah was something else.
It should also be observed that the wedding ring ritual is a kuffaar practice which Muslims should not imitate. We have our own sacrosanct and noble practices to follow. We are not in need of borrowing and aping the norms of the kuffaar in their marriage practices. Whilst it is permissible for the husband to give his wife a ring at any time, it is not permissible to observe any wedding ring custom.