There was low opportunity--which is one of the many reasons why it was so difficult. Means of earning money was up to the discretion of the slave owner. For example, slave owners could hire people out to do other labor, sometimes more skilled labor, like carpentry or working on a riverboat. He could elect to give some of the wages earning to the slave... or not. In Louisiana, slaves were not allowed to work on Sundays without being paid for it. So some slaves hired themselves out on Sundays and the 3 to 5 days they had off at Christmas/New Years. They might also work for their owner and be paid a fee for that, but of course the owner could pay only the tiniest wage. Also slaves needed their day off per week to tend their own small plots of land where they grew their subsistence food, and they needed money to pay for clothing and small amenities like blankets, shoes, a little extra food.