Yes but it's stupid...The only time putting a 50% sell on clause makes any sense is when a small team from a poor league sells a player to a club famous for developing players and selling them on. Paper nonsense I say.
It actually does make a lot of sense. Barca ripped them off and Arsenal were in a dreadful negotiating position. Since the entire thing revolved around how Fabregas was returning home, was always meant to be at Barca, etc. it makes sense that Arsenal put forward an argument whereby if that didn't pan out that way they were set to make something off it.
Scenario 1 (most likely, 95%): Fabregas signed for 35M, stays with Barca for aeons, Arsenal have got fecked and maybe some day receive a paltry sum when he is sold on as an OAP
Scenario 2 (current rumour, 5%): Fabregas doesn't work out so Arsenal want first refusal at a set price lower than that paid by Barca (player being older, hasn't worked, etc. why would they have to pay MORE???? unless they were being ripped off, of course). Obviously, Arsenal have to contemplate that maybe they won't even want/need him by then so also agree a 50% take on any sell-on fee.
That is, for any fee up to 50M Barca are better off selling back to Arsenal for less than they bought him for. If the fee is higher than 50M then the other options would be preferable and Arsenal get compensated due to their player going for more than 50M when they got rodgered to begin with having to sell him under duress for 35M.
What's illogical about all that? I can't see why Arsenal wouldn't want those clauses thrown in, nor why Barca would have even bothered with it back then. They got a world class midfielder, probably the best one not already at Barca at that point, for the same money Liverpool got Andy Carroll. Who cares what sell-on and buyback clauses the geezers want?