I think just trading with other countries. USSR had to trade with other countries to develop economy. Relations between animals and humans from close farms parody relations of USSR with other (non communist/western) countries.
For example soviet government sold many works of art from Hermitage collection to pay for industrialization. And in Animal farm they also sell something that got left from human ("tsar") times.
When the pigs trade with the humans, what does that symbolize in the russian revolution?
