[justshutupandtakeit]

There is nothing in Article IV, Section 3 of the constitution of the UNITED STATES which applies to secession. Not a line, not a word.

You must have it confused with the constitution of the Confederate States.

[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.