I agree with zach141. The flight to wherever you are going should be an instructional flight (gauging by how you are saying it), hence perfectly legal under the provisions in FAR 119. Now on the flight back, there is nothing "for sale" aboard the aircraft (you're not transporting persons or property for compensation), so this would not fall under the grounds of common carriage.

Let's even change the scenario. Imagine he rented (or is providing) the aircraft, and is willing to compensate you to fly him to the destination--no flight instruction is even intended. As long as he is not selling anything aboard the plane (like charging passengers to come along or carrying property for a customer) and you were never "holding out" your services, that would be private carriage. You flying the plane back and being paid for it isn't any different because, again, even despite the fact that he's not physically there, he's still not selling anything aboard the plane. Again, FAR 119 specifically excludes ferry flights (which is what you would be doing on the way back) from requiring an operating certificate.

I'm no lawyer, but it sounds legal to me.