It's a bit rich to say Arsenal have stitched City.
We know he's off, we are agnostic between City and Man Yoo, if we get something out of it by dealing with Utd (player swap, we did ask for Sterling I think previously) then it makes sense to do it with them. In consequence, hardly surprising all things being equal we allow Sanchez to choose the one he wants to go to. Given the reported salary thats being offered by United (which is higher than City's offer), it's not difficult to see why he's chosen United.
Transfer deadline day. £60m was agreed but Arsenal did fuck about that day in getting in a replacement and ended up in keeping an unhappy player.