money is great and all, but when you are not a selling club, you have to look at reasons why you would want to sell.
-player not happy (more often than not this is the main reason)
-offer is good enough that you can replace player sold with other players for same value/lower value.
now you might say that 50 million is god enough to land us a striker and another mid. you have to name names. cavani? napoli will want all of 50 million, then some. same will happen for jovetic or other high profile players.
then you have to replace vidal himself.
these issues would apply to all the important first team players