Peter stood there staring for a moment, tensed all over as though ready to run, rattish instincts demanding drastic action and yet unable to take it... but when the bag was offered, he seemed to collapse from the inside outwards. He had to stop being afraid. Or at least, to try.
He silently took one of the sherbert lemons, and held it in his hand for a moment. "Sorry," he said finally. "I know you weren't - it's - I meant, I don't want a magic hand. The healers sort of... patched it up... but I heard that I had one - or, well, I will have one? In the future?" he frowned in frustration at his own explanation. "And it's bad. So, I'd rather just put up with it."
He sighed. "The magic thing is.... well... I think it's my wand, more than anything. I've changed a lot since I was eleven, and it... well." He sighed and shook his head. He may as well be truthful about it. It wasn't like Dumbledore couldn't just ask anyone who would tell him.
"It doesn't work as well as it did before I started using Dark Magic," he explained, low. "I don't want to, anymore, but it seems to be all I can do, now."