His contract extension was automatic after promotion. Extra year and a wage rise. Didn't divulge that on the article that announced it but that's the truth. I expect we'll sell him next summer with two years on his deal for over £5m at least, fitness permitting.
Even with our dubious record of undervaluing players, if he's hit 15-20 goals in the Championship and still has two years left on his contract, I can't see him being allowed to leave for much less than £10 million. Strikers carry a premium after all.