Is retrieving part of the value of an account that was purchased and then restored to its original owner considered theft, knowing that the retrieved amount will be sold?

You bought an account you do not own, and its original owner reclaimed it, which is his right. It is not permissible for you to infringe upon his account or take anything from it without his permission, and his disbelief does not make it permissible to infringe upon his property. You must return what you took unjustly, and it is not permissible for you to sell it or dispose of it. If you do so, you are liable for it. The amount you paid to purchase the account should be reclaimed from the person who sold it to you, not from the original owner.