Well, no. South Carolina ceded the property on December 31, 1836 without any such qualifiers: Link
Work may not have been finished by April 1861 but it was certainly underway, with construction being performed under the supervision of an Army officer as late as December 1860 when Major Anderson transferred his command to the site. The transfer of ownership passed by the South Carolina legislature in 1836 was certainly valid, and ownership of Sumter could only be returned to the state through Congressional action. The claim that it was South Carolina's or that the troops were there illegally is flat out false.
Oops. You've done it again.
I think it's you who've done it. Again.
Not that POS Bob Huddleston again! His web site is full of lies and myths, something you know an awful lot about.