Much though I would like to agree, City have not handled this well.
Mancini spoke to millions of people after the Bayern game to say that Tevez was finished. They they fined him an amount in excess of the contractual maximum and announced it publically. I don't like it, but add those two things together and he does have a case for constructive dismissal if the club cancel his contract.
He can argue that the club's refusal to grant him exceptional leave in the circumstances was a 'final straw'.
Doesn't come down to general rights and wrongs. It comes down to detail and what can be proved or not.
The outcome of this will be a Compromise Agreement with Tevez leaving by mutual consent with an undisclosed payment (which will be a lot).