Is Fox's appointment illegal?

This letter to the GAO raises an interesting point that I hadn't considered.

Under 5 U.S.C. § 5503, in order for Mr. Fox to be paid for his services as Ambassador, his nomination would have to have been pending before the Senate on March 29th, when the Senate went into recess. Moreover, according to a separate statute, 31 U.S.C. § 1342, the U.S. Government cannot accept "voluntary services" from individuals except in an emergency....

In the case of Mr. Fox, however, it appears that the "voluntary services" prohibition would still apply because the position in question is a statutory entitlement with a fixed rate of pay that cannot be waived (Section 401 of the Foreign Service Act of 1980 mandates a fixed rate of pay for the position of Ambassador).

There appears to be a clear conflict between the pay restrictions as enumerated in Title 5 of the United States Code, which prevent Mr. Fox from being paid due to the circumstances of his recess appointment, and the "voluntary services" provision of Title 31 of the United States Code, which mandates that the United States Department of State cannot accept "voluntary services" for the position to which Mr. Fox has been recess appointed.