They could have done that purely by getting him to sign a pre contract. Only scenario that fits for me is the one where McLean has insisted on the transfer now thus gaining us a nice fee.....yes it is a generous thing for him to do but of course he will pocket a chunk of that transfer fee as is the norm plus he still will get a signing on fee from Norwich into the bargain.
As has been said also, they prob have a couple of players going out of contract who similarly to KM, have told Norwich they are leaving in the summer but at present are first team regulars who would keep KM from playing so best all round he gets a regular game with us for now.