My understanding here is the original poster would be on solid legal ground only if he shared the direct operating costs of the flight equally as if a private pilot carrying passengers.

OP did not mention this, but beware of accepting free PIC flight time as payment for anything. In the eyes of the FAA it is still payment even though no money changes hands.

Often you will see small aircraft owners approached to supply an airplane and a pilot. The owner can supply one or the other, but not both. If they supply the airplane it is a rental, which is legal, and if they supply a pilot it is ok also, but he cannot also fly the rental airplane or it amounts to Far Part 135 taxi service without a certificate.