Apart from the vexed question of whether souls in the spiritual sense actually exist and, if so, how exactly they differ from the more quotidian psychological conceptions of self like mind and personality, the metaphor of the Devil extending offers of power and prosperity in exchange for a financial transaction involving your precious little lonesome is nothing more than the concrete expression of distinctly un-Satanic panics and preoccupations. I’ve covered the matter in this post here, if you want to take a look.
The reputed “Great Deceiver” has little use for fiduciary duties bound through contractual obligations. Assuming his reputation for playing folks false holds any water at all, Satan would probably prove uniquely poor at discharging such duties at any rate.
If you’re desperate or deluded enough to want to sell to another all that which is most distinctly yourself, you’re already far, far away from placing the kind of value on self that has everywhere and at all times been the mark of the Satanist, whether born or made. You should probably take a step back, gather yourself, and go do some serious “soul-searching” (or even seek professional mental health care?).