Boxer's death is the end product of all the different ways the pigs have been abusing and manipulating the other animals. In his life, Boxer was the most valuable laborer - he was the strongest, most hard-working, and the most ignorant. Not only did he help the pigs build their projects, he also did some propaganda on the side. Readers would think that the pigs would treat him much better, but they don't.
To have the pigs sell him off for money is to show just how little the pigs actually care for the laborers. To the pigs, Boxer was only worth keeping around so long as he could continue working for them. It is a very clear criticism of how the Soviets ended up treating the poor many times worse than the old regime did.
By the way when you write this down, make sure to reword it. Otherwise you could be caught for cheating.
