It other words since when South Carolina seceeded there was no provision for the disposition of "foriegn property" within the borders of that State. Therefore, the property would automatically return to the States for disposition by treaty or other agreement. Since Fort Sumter was unocupied at the time of South Carolina's secession it was therefore abandoned/unclaimed property. The occupation of that property as a military installation by Major Anderson would have been an Act of War.
In other words an invasion. It was no differenent than if the Russians had sailed into Charleston Harbor and taken up residence at Fort Sumter. Just because The Government of the United States and The State of South Carolina had a former agreement prior to December 20th, 1860, was no reason to assume ownership. That agreemnet was no longer binding once South Carolina declared its Independence from the United States. Any disposition of propperties at that time should have properly been settled in "Divorce Court" and not by military forces.