Nobody who has not been officially appointed as a contracting officer with a warrant or as a GPC card holder can legally obligate federal funds. The IG office cannot obligate funds until and unless a person who can officially obligate funds actually does so. The invoice can be paid through the ratification process if the requirements at FAR 1.602-3 are met. The fact that the services were performed in the current fiscal year simplifies things a bit. And of course, be certain to work directly with your activity's legal counsel on the matter.