The case of Thiago Alcantara is well known: he left the club of his life, Barça, at just 22 years old for a ridiculous sum (€25m). This was just as everyone started to believe he was the natural replacement for Xavi or Iniesta. If he had played two more matches, his clause would have risen to €90m and as such become unattainable for rival teams.
Seven years later and nobody knows who was responsible for that unsavory parting of the ways. Some people still want us to believe that Thiago forced the move through by himself. However it's also true that it was very easy for Barça to stop him from leaving and it's incomprehensible that they didn't. In the end, both sides lost. Away from Barcelona and outside of the system which helped him develop, there's a feeling that Thiago didn't quite scale the heights expected of him when he rose through the ranks at La Masia.
And without Thiago, Barça broke their conveyor belt of talent which linked La Masia to the first team and still haven't found the right person to replace the gap left in midfield by the golden generation. Bayern are now willing to let the Spaniard leave the club. Barcelona are also ready and willing to make an offer for the player who once belonged to them. Sources close to the German club say they'd accept a bid around the €50m mark.
However, the situation creates another dilemma. Could Barça re-sign one of the guys who decided to leave, one in which they didn't even have an option to buy back the player (as was the case for Gerard Pique)? Is it preferable to sign someone who came up through La Masia? Or, as Xavi said, would it be unacceptable because it would send a devastating message to those who decided to stay? This is the dilemma behind why Barça, who are searching for a central midfielder, haven't made an offer for Thiago yet.