Snape watched Al for a long moment, and when he was satisfied that the spells the boy was casting at the bookshelves weren't different from his previous repertoire, he nodded to himself and strode back out into the warren of Potions and Dark Arts research labs deep under this, the most far-flung outcrop of the castle.
From time to time he'd sneak silently back into the study, picking this up and dropping that off - and only incidentally checking on Al - but every time Al's behaviour was perfectly innocent.
He hmmed and studied the shelves. "I suppose you can continue our arrangement into the new year." He added belatedly, "If coming to the castle isn't an inconvenience."