Is the money reimbursed from e-wallets as a result of paying bills or recharging, which is offered as a percentage or a fixed amount, considered usury (riba)?

Digital wallets cannot be ruled upon except after understanding their nature of operation and their compliance with Islamic law. If the wallet offers a discount for paying electricity bills, and you do not have money in the wallet, then there is no harm. If you do have money in the wallet and it is invested in a permissible investment, then there is no harm in taking the discount as a gift for the investment. However, if the money is not invested, or if it is invested in an impermissible investment, then this is considered a loan, and it is not permissible to accept a gift for it, based on the saying of the Messenger of Allah, peace and blessings be upon him: "If one of you grants a loan and the borrower gives him a gift or carries him on a mount, he should not ride it or accept it, unless there was a prior custom between them."

As for charging the wallet with 100 and receiving 50 in return, this is not permissible. It is either an interest-based loan (riba) or a corrupt partnership. If the money is not invested, it is a loan on which taking interest is forbidden. If it is invested, then a percentage of the profit must be agreed upon, not a fixed amount or a percentage of the capital, along with not guaranteeing the capital. Otherwise, it becomes a trick to disguise an interest-based loan. The conditions for permissible investment are that it must be in a permissible field, with no guarantee of the capital, and an agreement on a known percentage of the profit, not of the capital.