(a) That's right. The amount that A is claiming appears to be a direct loss. Direct loss is defined as loss that follows naturally from the breach, according to the usual (or normal) course of events.
In the present case, the lower value of the bull follows naturally from the fact that it is not a breeding bull, which is what it was sold as, because higher prices are paid for breeding stock than for slaughter stock. The loss suffered by A is the difference between what he paid, and the value of the animal he has received. As a slaughter animal, the bull is worth $500. Assuming that A paid the market value for the bull, his loss is $4,500, being the difference between that value and the value of what he received.