Watson could walk away without looking back if you consider that Amphastar only received $4.5 mil upfront and $5.5 mil upon FDA approval from Watson (Andrx originated the deal).
If they don’t intend to see this through to an eventual launch, WPI will presumably try to get something back from Amphastar rather than walking away from the program and “leaving the keys at the bank.” Even if all they got back was a nominal amount, it would avert the loss of face from abandoning the program in return for nothing.