24 C.F.R. § 850.155
Assistance provided under this part shall constitute a debt of the owner (including its successors in interest) to the grantee, and shall be secured by a mortgage or other security instrument. The debt shall be repayable in the event of a substantive, uncorrected violation by an owner of the obligations contained in paragraphs (b), (c), (d) and (e) of § 850.151 . The instruments securing this debt shall provide for repayment to the grantee in an amount equal to the total amount of housing development grant assistance outstanding, plus interest which is determined by the Secretary by adding two percent to the average yield on outstanding marketable long-term obligations of the United States during the month preceding the date on which assistance was made available. The amount to be repaid shall be reduced by 10 percent for each full year in excess of 10 years that intervened between the beginning of the term of the owner-grantee agreement and the violation.
24 C.F.R. §850.155