John,
I think that is different from what we normally think of as slavery. I can understand the situation that the couple found themselves in. They are able to get out of the servitude by returning the land to the former owner. I was indentured as an apprentice carpenter for a four year period. I had one of my great grandfathers who learned the tim trade back in the 1830's the same way.
I had an uncle who came from Sicily and migrated to Venezuala after WWII. He developed a method of extracting chemicals and nutrients from sea weed, obtained a land grant from the government and went back to Sicily and brought many families to South America. They were paid a small wage and given a piece of land with a house on it in exchange for their labor in his enterprise. After a period of time (five to eight years) the land was theirs and their wages were increased. That seems fair to me and it seems that is what you ancestor was doing.
Frank