Does Interim Director have the final call authority here or does Supervisor? If Supervisor, I'd relay that Interim Director requested the work directly and ask for direction on how to proceed. If it's Interim Director's call, I'd send one last email documenting the low level of risk and the increased cost and ask for a final decision. If Interim Director decides he/she still wants the extra work with that information, there's not much else to be done other than get on with the extra work. At least if someone questions it later, you'll have documentation that it was Interim Director's call, not yours.