You don't really put future salary costs on the book like that. His book value will (assuming linear depreciation) be the cost of acquisition (price, one time fees to agents, player etc.) times the fraction of time left on his contract. Sure, they would save future wage expenses by selling him, but accounting-wise it is not recorded that way. Was the price 110? And 3 years left of a 5 year contract? If so, he will probably be on the books at about 65 million €. But, i don't really think his book value matters too much in this case, United are really well off financially, so even IF they would sell him, his price would probably depend on the market: how many similar players of type/quality are potentially on the market, and how many clubs are looking at a player like that, and in turn what are these clubs willing to pay.