Is taking a percentage (one pound for every five pounds) in exchange for breaking down large currency denominations on public transportation considered usury (riba)? If so, do the earned funds become unlawful, and what is the ruling on taking the change in this situation?

It is not permissible to sell paper currency for less than its value in metallic currency, because currency is one genus, and thus requires simultaneous exchange (al-taqabud) in the same session and equality [in value]. Paper and metallic currencies take the place of gold and silver, and are considered distinct categories based on their issuing authority. Whoever repents, Allah will accept his repentance. Past transactions conducted before knowledge of the religious ruling are forgiven. Taking the change when using transportation is permissible, and leaving the change as a trust with the transportation provider is also permissible.