Barely. The problem isn't Mali specifically, but the interaction between civs that both need either 0 slaves or have excess slaves. The problem is that the code doesn't allow to set different values for selling and buying. Currently, the AI is set to value slaves as 0 if it doesn't need them. The point is that then the AI won't offer you anything in return when you put a slave on the trade table. That's problematic however when it is the AI that has the slave.