To add to the above, the parcel of land is also a sign and a symbol for the economic system and class stratification of Wang Lung's community. As Wang Lung pays the agent for the parcel of land, he watches the way the agent sweeps up the silver coins with no real regard--the agent then comments that the money will be enough to buy opium for a next few days for the master. But Wang Lung had worked so hard for those few coins, and to him, they meant much more than they did to the agent.
Also, if the line is read as a rhetorical tool used by the author, then it also foreshadows all the trouble that Wang Lung will meet by the end of the novel. Money eventually corrupts Wang Lung's good intentions and his family, but at this point in the story, he has no way of knowing this and it is only a hint given by the author.