The head of each such department or agency, hereinafter in this subsection called the trustor, is authorized to set aside a part or all of any obligations held by the trustor and subject them to a trust or trusts and, incident thereto, shall guarantee to the trustee timely payment thereof. The trust instrument may provide for the issuance and sale of beneficial interests or participations, by the trustee, in such obligations or in the right to receive interest and principal collections therefrom; and may provide for the substitution or withdrawal of such obligations, or for the substitution of cash for obligations. The trust or trusts shall be exempt from all taxation. The trust instrument may also contain other appropriate provisions in keeping with the purposes of this subsection. The Association shall be named and shall act as trustee of any such trusts and, for the purposes thereof, the title to such obligations shall be deemed to have passed to the Association in trust. The trust instrument shall provide that custody, control, and administration of the obligations shall remain in the trustor subjecting the obligations to the trust, subject to transfer to the trustee in event of default or probable default, as determined by the trustee, in the payment of principal and interest of the beneficial interests or participations. Collections from obligations subject to the trust shall be dealt with as provided in the instrument creating the trust. The trust instrument shall provide that the trustee will promptly pay to the trustor the full net proceeds of any sale of beneficial interests or participations to the extent they are based upon such obligations or collections. Such proceeds shall be dealt with as otherwise provided by law for sales or repayment of such obligations. The effect of both past and future sales of any issue of beneficial interests or participations shall be the same, to the extent of the principal of such issue, as the direct sale with recourse of the obligations subject to the trust. Any trustor creating a trust or trusts hereunder is authorized to purchase, through the facilities of the trustee, outstanding beneficial interests or participations to the extent of the amount of the trustors responsibility to the trustee on beneficial interests or participations outstanding, and to pay the trustors proper share of the costs and expenses incurred by the Association as trustee pursuant to the trust instrument.