(b) the deceased was concerned in a fiduciary capacity imposed on him otherwise than by a disposition made by him and in such a capacity only; and references to a gift in this Act shall be construed accordingly: Provided that where the disposition was made on the part of the deceased for partial consideration in money or money' s worth paid to him for his own use or benefit, the value of the consideration shall be allowed as a deduction from the value of the property for the purpose of estate duty.