Why didn't Mama want Stacey working for Mr. Harrison, even though he could earn $5.00 that they could put towards their cotton tax?

Mama cites that her husband doesn't break his back on the railroad, so her son can work for a white man. She tells Stacey that if he wants to work, there's four-hundred acres of land waiting for him...... land that belongs to the family. She thanks Mr. Harrison for the offer and tells him that her son is needed at home.