So city bid 36, sporting rejects and pays him 2. The bidding continues with 37, 38, 39... When sporting rejects a bid of 50, they'll pay the player 30 altogether. When the bidding reaches 60, they'll owe him 50. If they accept sporting makes 60 - 50 = 10. If sporting accepts 70 they make 0, and after that they'll depart with him at a loss.
Conclusion: the russian figure of 50 seems fair to all parties, unless the contract is more complicated than our portugese friend has understood.
Caveat: I haven't double checked the calculations since my wife is in labour..