as long as the cat was not a pedegrie there should be no problem expecially as it was not intentional the law states that if a cat comes onto your property youre allowed to dispatch it as long as its not a pedegrie so walking across a field and a acciedent like that happens there should be no recopucions (did the dog do well, and check that non of the cats claws dislodged and embedded themself into your dog this often happens, they can cause nasty infections if left unattended)