Is transferring money via "Western Union" considered usury (riba) if the sender pays an additional commission for the amount to be sent, and what is the expiation for whoever has done so in the past?

There is no objection to transferring money through exchange companies. The additional amount paid is a service fee for delivering the sum, and it is permissible as long as it is known and specified at the time of the contract. Its variation depending on the amount transferred does not affect its permissibility. The "qabd" (possession) is realized upon receiving the transfer receipt at the contracting session. The money transfer is considered an authorization for the company to deliver the amount, whether in the same currency or another. In the first case, it is a pure agency, and in the second, it involves both currency exchange and agency.