The application for a loan to be guaranteed hereunder shall be submitted to the Secretary for approval. The Secretary may review each loan application individually and independently from the lender. Upon approval, the Secretary shall issue a certificate as evidence of the guaranty. Such certificate shall be issued only when, in the judgment of the Secretary, there is a reasonable prospect of repayment. No loan to an individual Indian may be guaranteed or insured which would cause the total unpaid principal indebtedness to exceed $500,000. No loan to an economic enterprise (as defined in section 1452 of this title) in excess of $250,000, or such lower amount as the Secretary may determine to be appropriate, shall be insured unless prior approval of the loan is obtained from the Secretary.