If you offered to fly a manager and he rented the aircraft and you were asked to fly it for him for no fee.
Is this pushing the boundary?
What you probably mean is you rent the aircraft and the manager pays you for the rent... which is purely illegal out of an AOC. It all depend on who has the operational control of the aircraft. If you rent it from an airclub, the aircraft is under the airclub's operational control and should only be "rented" to members/pilots. In such case, being paid to rent the aircraft on behalf of a third party is most probably illegal especially if you fly a "manager" for business, even if you're not paid for flying the aircraft.
This is very different from the situation in which an individual has the operational control over the aircraft (ownership, leasing), in which case you can be paid to fly this individual for his private use, without needing an AOC.
This is NOT a commercial operation. As the requlations stand you do not require an AOC. You should have an operating / operations manual.
What for?