They don't, remember reading that Chelsea made numerous mistakes by not having buy back clauses inserted in contracts of those they sold
De Bruyne, Lukaku, Sturridge and Matic (who they ended up spending a shed load to buy back due to no clause)
It was reported a few weeks ago that there are are some clauses in the original contract and that Chelsea would be able to use these clauses to reduce any fee. Whilst I wouldn't be surprised about there not being a formal buy back option I wouldn't be surprised if Chelsea have a first option as indeed they had with Matic also,it is almost the norm for a standard clause requiring a% of future transfer profit to be paid . It's all too easy to look at a transfer in isolation but you have to try and see a bigger picture and even then we in the main have no idea what players have been promised verbally or indded in writing
For instance Matic originally cost Chelsea£1.5 million later he was part of the deal to take sideshow Bob the other way meaning to sign Luiz in effect Chelsea paid about £21million plus Matic .
Subsequently Matic was brought back for £21million so an outlay of about £44 million. Sideshow bob was sold for a reported £50million. So at this time if you look at at these basic transactions they have Matic and a small profit.
Lukaku isn't going to be sold for £65 million if he leaves it will be top £50 million. By the time you take out things like agents fees, 5% going to the FA and another 5% to the player, unless he formally asks for a transfer and if the £28 million was indeed right then at most I would put any profit as being around £10-15 million