Chelsea have the goalkeeping position locked for say 4-5 years with Cech. Courtois would give 15, so it's a stupid long-term idea to sell him but If the club is unwilling to replace Cech now, they might as well sell Courtois with a buy-back option because as you said, at one point Chelsea will want Courtois back 100%. I don't agree that it doesn't make sense for a club to try and get Courtois with a buy-back clause included. The club will be getting one of the best keepers in the world and worst case, if Chelsea do take him back down the line, they'll reinvest in another goalkeeper.