[Libertarianize the GOP]

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.

That very clearly empowers the Government to negotiate all territorial and property issues that would occur within a mutually agreed upon secession.

[justshutupandtakeit]

That is not an accurate understanding of the meaning of this phrase. It is speaking to the Territory of the United States not yet formed into States i.e. the Northwest Territory which later became Ohio, Indiana, Illinois, Michigan etc.

It is not speaking about already formed states but territories. Certainly you do not believe that States are the property of the United States do you? By that interpretation the Congress could have sold South Carolina to France or Spain. Such an idea is ridiculous on its face.

In order for the suicidal act of secession to be legal the constitution would have to be amended since neither States nor Congress can legislate any changes in it.