; or (B) by purchase of outstanding obligations at market price. (3) Sale of obligations Any obligation acquired by the Fund may be sold by the Secretary of the Treasury at market price. (4) Credits to fund The interest on, and the proceeds from the sale or redemption of, any obligations held in the Fund shall be credited to and form a part of the Fund. (d) Transfers of Amounts (1) In general The amounts required to be transferred to the Fund under this section shall be transferred at least monthly from the general fund of the Treasury to the Fund on the basis of estimates made by the Secretary of the Treasury. (2) Adjustments Proper adjustment shall be made in amounts subsequently transferred to the extent prior estimates were in excess of or less than the amounts required to be transferred. (e) Acceptance and use of donations (1) In general The Secretary may accept for the Government a gift of any of the following to provide assistance under section 404: (A) money; (B) an obligation of the Government included in the public debt made only on the condition that the obligation be canceled and retired and not reissued