Repayment of all or part of the principal, which is the loan plus the administrative fee, may be deferred during a development period not exceeding 10 years within the maximum 50-year repayment period, when, in the department’s judgment, the development period is justified under the circumstances. Interest on the principal shall not be deferred. Repayment of principal which is deferred during a development period may, at the option of the supplier, be paid in annual installments during the remainder of the loan repayment period.