“ And be it further enacted, That all persons held to service or labor under the laws of any state, and who at any time since the sixteenth day of April, anno Domini eighteen hundred and sixty-two, by the consent of the person to whom such service or labor is claimed to be owing, have been actually employed within the District of Columbia, or who shall be hereafter thus employed, are hereby declared free, and forever released from such servitude, anything in the laws of the United States or of any State to the contrary notwithstanding. ”