(2) For the purposes set forth in section 1716(a) of this title , the corporation is authorized, pursuant to commitments or otherwise, to purchase, service, sell, lend on the security of, or otherwise deal in mortgages which are not insured or guaranteed as provided in paragraph (1) (such mortgages referred to hereinafter as “conventional mortgages”). No such purchase of a conventional mortgage secured by a property comprising one- to four-family dwelling units shall be made if the outstanding principal balance of the mortgage at the time of purchase exceeds 80 per centum of the value of the property securing the mortgage, unless (A) the seller retains a participation of not less than 10 per centum in the mortgage; (B) for such period and under such circumstances as the corporation may require, the seller agrees to repurchase or replace the mortgage upon demand of the corporation in the event that the mortgage is in default; or (C) that portion of the unpaid principal balance of the mortgage which is in excess of such 80 per centum is guaranteed or insured by a qualified insurer as determined by the corporation. The corporation shall not issue a commitment to purchase a conventional mortgage prior to the date the mortgage is originated, if such mortgage is eligible for purchase under the preceding sentence only by reason of compliance with the requirements of clause (A) of such sentence. The corporation may purchase a conventional mortgage which was originated more than one year prior to the purchase date only if the seller is the Federal Deposit Insurance Corporation, the Resolution Trust Corporation, the National Credit Union Administration, or any other seller currently engaged in mortgage lending or investing activities. For the purpose of this section, the term “conventional mortgages” shall include a mortgage, lien, or other security interest on the stock or membership certificate issued to a tenant-stockholder or resident-member of a cooperative housing corporation, as defined in section 216 of title 26 , and on the proprietary lease, occupancy agreement, or right of tenancy in the dwelling unit of the tenant-stockholder or resident-member in such cooperative housing corporation. The corporation shall establish limitations governing the maximum original principal obligation of conventional mortgages that are purchased by it; in any case in which the corporation purchases a participation interest in such a mortgage, the limitation shall be calculated with respect to the total original principal obligation of the mortgage and not merely with respect to the interest purchased by the corporation. Such limitations shall not exceed $417,000 for a mortgage secured by a single-family residence, $533,850 for a mortgage secured by a 2-family residence, $645,300 for a mortgage secured by a 3-family residence, and $801,950 for a mortgage secured by a 4-family residence, except that such maximum limitations shall be adjusted effective January 1 of each year beginning after the effective date of the Federal Housing Finance Regulatory Reform Act of 2008, subject to the limitations in this paragraph. Each adjustment shall be made by adding to each such amount (as it may have been previously adjusted) a percentage thereof equal to the percentage increase, during the most recent 12-month or 4-quarter period ending before the time of determining such annual adjustment, in the housing price index maintained by the Director of the Federal Housing Finance Agency (pursuant to section 4542 of this title ). If the change in such house price index during the most recent 12-month or 4-quarter period ending before the time of determining such annual adjustment is a decrease, then no adjustment shall be made for the next year, and the next adjustment shall take into account prior declines in the house price index, so that any adjustment shall reflect the net change in the house price index since the last adjustment. Declines in the house price index shall be accumulated and then reduce increases until subsequent increases exceed prior declines. The foregoing limitations may be increased by not to exceed 50 per centum with respect to properties located in Alaska, Guam, Hawaii, and the Virgin Islands. Such foregoing limitations shall also be increased, with respect to properties of a particular size located in any area for which 115 percent of the median house price for such size residence exceeds the foregoing limitation for such size residence, to the lesser of 150 percent of such limitation for such size residence or the amount that is equal to 115 percent of the median house price in such area for such size residence.