I might not be clear on the point your making, so my apologies if I've misunderstood! I'm not sure how the issue is the use of proceeds from a sale of assets, in that in assuming that the slave owner would be selling assets (be they slaves or otherwise) to raise capital for whatever use was required (be it to pay debt, invest, purchase goods, etc.).
I agree that certainly the sale of any asset could then decrease the wealth of the planter, depending on the use of the capital raised. If it was done to re-invest however, it could actually raise the overall wealth of the investor. If used to pay debt or (as an example), it would certainly reduce that individual's overall wealth.
I don't think there's a lot getting lucky regarding the sale of slave, as arguably the price of slaves (and their labor value) consistently trended upwards over the period of 1804-1860. Which shows just was a good, but tragic, investment slaves were. Here's a graph that shows the increase in slave prices in the US:
Average Price of a Slave Over Time
View attachment 214658
Source:
Historical Statistics, Table Bb212. Average Slave Price.
Absolutely agree that slave owners would hire out slaves if they were surplus to requirements or for profit. Which would be a great way to gain income without actually losing the asset. Which is something that many non-slave owners might not be able to do with many of their assets.
I suppose my point is that slaves provided slave owners with a remarkably useful and valuable asset which helps to demonstrate, in my view, the terrible flexibility of the slave system.