So here is the thing - WBB could argue that Durant modified the contract in a way that made the contract invalid by retiring before playing football.
Essentially the initial contract was - We pay you 70k on Jan 15, you come play football for us, during which we pay you an additional X dollars per year.
Durant modified the contract by retiring before playing - WBB pays me 70K on Jan 15, I retire before playing football for the club.
The problem for Durant is that contracts in Canadian law require that both parties receive consideration for a contract to be considered valid. By modifying the contract and retiring before playing Durant has essentially made the contract invalid (WBB received no consideration in return for his signing bonus). The only thing Durant could argue is that WBB received his not playing for another team in return for 70K.)