Any evidence of guaranty or insurance issued by the Secretary shall be conclusive evidence of the eligibility of the loan for guaranty or insurance under the provisions of this chapter and the amount of such guaranty or insurance: Provided, That nothing in this section shall preclude the Secretary from establishing, as against the original lender, defenses based on fraud or material misrepresentation or bar him from establishing, by regulations in force at the date of such issuance or disbursement, whichever is the earlier, partial defenses to the amount payable on the guaranty or insurance.