Just some thoughts on this.
If it were you, you made a sacrifice to restructure your wages to help the club and now they try to screw you over and throw you out the door without paying all those back wages, would you willingly help them again by leaving or would you double down and demand they either pay the back wages or you will stay? Hard to infer anything into this more than that. He may or may not have wanted to come, but that is all irrelevant now as 17 million pounds are at stake here. He could just stay and screw them over by taking the back wages and leaving on a free and sign even higher wages at the end of his contract. I don't see anything wrong with this. He has the moral high ground AND the bargaining leverage. Barca will lose their transfer plans and they also cannot afford to pay so much to someone to just bench him in the reserves for the whole season.
It also isn't unreasonable for Man Utd to know that FDJ is getting shafted and offering him somewhere to go which he is open to joining, but pending the back wages being resolved. Unfortunately, it now doesn't look like it will be resolved, which means the deal will likely fall through. Really not an issue of the club that Barca are run by right real pricks. Now it's a question of how desperate Barca will get and if they can broker a compromise deal with FDJ after all this bad blood. I'm pretty sure Man Utd already put a deadline on this and will move on if this isn't resolved by then. If they do, then fair play and they tried their best. Move on and get other targets. Put this circus behind us.