Source? He's still on loan. There's been no news about him being sold as of yet from any dependable source.
Anyways, given that Nani actually likes this club and has stated that he left just to rejuvenate himself and his career, I wouldn't see why he would really want to leave. He likes to play for us, he'll be playing under a more reputed manager in van Gaal who's also more personal and honest to his players, he'll be playing alongside star players like di Maria, Rooney, de Gea, etc., and he'll be coming back to a club on the rise. Nani, first, will have to listen to what van Gaal wants from him and whether van Gaal will give him chances or not based on the reports he received about Nani's loan spell and through observing him during pre-season.
Given that Nani's been consistently good for Sporting so far, I wouldn't think that van Gaal will easily let him go. However, I wouldn't rule out van Gaal buying Depay at this point given how strongly
@NL Max feels about this Depay coming to us. If van Gaal does buy another winger, and if he doesn't let go of 1-2 wingers, Nani will most likely reconsider his future and look to leave, and I'm sure that, after a good loan spell, he'll have several good clubs looking for him all around the world.