No! Can't Syaoran return what he took? Argh! no, no, no..... *ears droop*
It's possible that he
could, but I doubt Fai would want him to. Fai's probably not interested in ripping the eye out again eating it out - he'd probably be more interested in getting their Syaoran back than getting his eye back. I doubt he's as obsessed with it as his fangirls are.
He might like to have the magic back, but he'd probably be willing to let Syaoran have half of it (since he didn't really want to use it anyway). Syaoran would probably want him to have the magic back, as might Kurogane and Sakura, but Fai would probably insist that he was fine in the Fai-ish way of his.
This all assuming that clone!Syaoran returns to normal, which I'm not seeing as a sure thing right now. If we end up with real!Syaoran instead, he wouldn't have the eye at all, though if the clone were destroyed he might be able to give the magic back. Even then I don't think Fai would be keen on eating pieces of his fallen comrade (oh god, I'm grossing
myself out). Maybe the magic would return on its own if the clone was destroyed? I don't think that would restore Fai's eye, though.