best case scenario : the 400 dollar check dont bounce and you never hear another word from either party.

worst case scenario : the "owner" of the property drags both you and the sign company to court for damages to his property in the form of removing or dismantling the framework that would hold a sign or billboard.

this might have been an improvement made to the owners building by a tenant or this might have been a structure that was in place before local building codes were changed and it was "grandfathered" in as long as it was intact.