In the mid 1800s and even before then, slave owners would buy slaves using their money. Sometimes they would go to an auction after a ship came in of slaves and bid on slaves. Many times families were split apart including the children because different slave owners would purchase different people in the family. Most farmers did not own slaves because they were expensive and they worked their own farms.
So the slave owners would be making a lot of money and a large plantations. They would purchase slaves for a few $100. Sometimes a healthy strong slave might cost several $100. So if a slave ran away plantation owners would do anything to get that slave back. It would cost them a lot of money to buy another slave.