What is the ruling on working in the transportation of currency within the country before it is converted and sold, and is mutual possession (قبض) a condition in such transactions?

Possession is a condition when exchanging currencies. The issuance of a certified check or an authenticated document by the exchanger serves as a substitute for possession.

As for your work in transporting currencies for exchangers, it is a permissible lease for a lawful benefit. You are not required to verify their adherence to the exchange conditions in every transaction, as long as the presumption is that they comply.

However, if the presumption is that they do not comply, then it is not permissible to work with them, due to the assistance it provides in sin. There is no difference in this regard between a usurious exchanger and a usurious bank.