(a) The land contract guarantee constitutes an obligation supported by the full faith and credit of the United States. The Agency may contest the guarantee only in cases of fraud or misrepresentation by the seller, in which:

(1) The seller had actual knowledge of the fraud or misrepresentation at the time it because the seller, or

(2) The seller participated in or condoned the fraud or misrepresentation.

(b) Loss claims also may be reduced or denied to the extent that any negligence contributed to the loss under § 763.22.