Another facility can't give any "control instructions" in somebody else's airspace. So, no, Potomac couldn't give permission to land. Just like they can't/don't at uncontrolled airports.

If Potomac would have given some "permission", and a Boeing landed on an unlit fuel truck on the runway (that a terrorist could have put there), well, you can imagine the mess.

I'm with you; I would have landed somewhere else.