I have a deleted file archive database that stores the ID of the file that was deleted, I want the admin to be able to restore the file (as well as the same ID for linking
Should you instead be setting the identity insert to on within the stored procedure? It looks like you're setting it to on only when changing the stored procedure, not when actually calling it. Try:
ALTER procedure [dbo].[spInsertDeletedIntoTBLContent]
@ContentID int,
SET IDENTITY_INSERT tbl_content ON
...insert command...
SET IDENTITY_INSERT tbl_content OFF
GO