Casemiro may not be able to leave, because the problem is that United is paying him way too much, no team would pay him that much. So deal has to be for less than what he is getting paid for, meaning that whoever gets Casemiro, will only pay part of his salary and not all of it. I am not sure if that would be a good idea, because United wants to get rid of 100% of his salary. This is why they couldn't sell him, because nobody would agree to pay him what he is getting paid, and this is why he is not getting loaned out.
United will only be able to loan him out, if they agree to pay half of his salary just so he would leave, and even if not half of his salary, United needs to realize they will have to pay SOME of his salary to get him away. This is why the deal isn't done, they could have done it for European teams too if they agreed to it, but they realized they couldn't, so now it's a problem. Plus, he hasn't played a good game in over a year, which makes no team really want him that much.
Well Casemiro is still a vibrant player, I won't be surprised if a club comes to him on loan or outrightly buy him despite his huge weekly wages, this Saudi guys aren't playing with you whyen they are coming for a player, so in the aspect of the salary I don't think any Club that really wants him would look at that, Manchester United just want to get rid of him, probably because he doesn't fall into the specification of what Amorim needs for his team, just the way he did for Rashford.