I think this is a golden oppertunity for Chelsea to show some respect for the league this summer. Its a rare one wo they best take up the chance!
Right if they are keeping lukaku then go ahead keep him. He is a fantastic player and would score for fun in their team. NOw if they decide for whatever reason they want to sell him then they could do the following;
Court him around, name their price and see how many people will pay it, say it is 20 million. If teams say we'll take him for that, come to ourselves and west brom and go right, 15 million and you can have him, if not he will go to dormund or wherever for 20. That way, Chelsea can pay respect to the teams that have loaned him but cannot over bid for his services and worse case scenario is that they sell him for the 20 and replace him anyway.
I just dont see them selling though, he is part of a select group of players that they really wanted to sign and have kept on since e.g mikel. i believe chelsea will keep him and develop him into a superstar.