Maybe Van Gaal is prepared to let him go, as he's realised his mistake in signing him/doesn't think Di Maria is going go turn things around.
People could argue that Di Maria doesn't fit the system/philosophy and even though this might be true to some extent, I don't think it was the system/philosophy's fault that for the major part of last season, he was garbage. Yes, he made a double digit of assists, but some of the goals from these assists happened thanks to the goal-scorer and not the pass. Di Maria's cross for Rooney's goal vs Villa at OT was actually a poor one and even though his pass to Mata was very good at Anfield, it wouldn't have been a goal if Mata didn't try that spectacular shot. I'm not trying to diminish his contribution last season, I'm just saying that some of these assists were quite fortunate and his season was shite as a whole. On the other hand, he proved that he could be effective even when he's off form, which only shows that he's a world class player.
I think he'll be better next season if he stays as he'd have a full season in the league behind him, but it's clear to see that he's unsettled IMO, despite his 2 interviews in which he said he wanted to stay and win titles with United. He was hardly going to say ''I'm leaving'', as it's never the right tactic and could receive the backlash of the fans if a move doesn't materializes.
I was actually certain that he was going to stay, but with all these French souces basically saying the same thing + the Guardian and Ogden's reports, it really looks like that we're prepared to sell him. Whether that's because Van Gaal doesn't think Di Maria would turn things around or Di Maria's family wants a move away, is another matter. I think it could be both.