Not only beyond the President's power but also beyond the power of Congress to legislate slavery.
All legislative powers are enumerated in the Constitution and the power to legislate slavery is not among them. Powers not granted Congress are left to the states or the people.
The Constitution did not grant Congress legislative powers regarding slavery, voting, abortion, marriage, flag burning, or to prohibit the citizens from any endeavor in their pursuit of happiness.
The Constitution did provide an amendment process to add, or subtract, powers of legislation. The 13th amendment took the power to determine if a state would be free or slave away from the state and added it to congress’s legislative powers. The amendment itself is not legislation but it provides Congress with the power to enact legislation regarding the amendment. Section 2 of the 13th amendment adds the legislative power … Section. 2. Congress shall have power to enforce this article by appropriate legislation.
Of course territories were controlled by congress. In territories Congress has plenary power, just as the state governments have plenary power over their territory. The Northwest Ordinance banned slavery, and was passed by the Congress under the Articles of Confederation, with many of the members of the constitutional convention voting for it.
The Congress shall have power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Per Article 4 of the US Constitution. Of course this means that congress has authority to forbid slavery in any new territories.