; and (2) not more than 30 percent may be issued as preferred securities under subsection (a). (c) Federal Financing Bank The Federal Financing Bank may acquire a debenture issued by a participating investment fund company under subsection (a). (d) Purchase and guarantee by SBA (1) In general The Administration may purchase or guarantee the timely payment of all principal and interest as scheduled on debentures or preferred securities issued by participating investment funds under subsection (a), subject to such limitations and regulations as the Administration may prescribe. (2) Full faith and credit The full faith and credit of the United States is pledged to the payment of all amounts which may be required to be paid under any guarantee under this subsection. (e) Third-Party debt The Administrator— (1) shall not permit a participating investment fund having outstanding leverage to incur third-party debt that would create or contribute to an unreasonable risk of default or loss to the Federal Government