According to the fundamentalist Christian theology of my upbringing, souls can not be sold. No one, except for God and the soul's owner, can direct what happens to it. Your eternal destination is contingent upon you (and only you) going through the conversion / salvation process. If you do so, then Heaven awaits. If not, then your neverending vacation will be toastier. And the conversion process can be done solo. While pastors or missionaries or overbearing parents may be present for it, they don't have to be. No human intercession can save you once you're dead, nor can any human damn you in any way worthy of a damn.

Based on this theology, OP may want to sue their friend for fraud. I'm assuming this transaction was documented, with a signed receipt. Preferably notarized. However, I'm not sure how fraud could be prosecuted when the defendant was a minor at the time.

Disclaimer: I'm looking at this case via the theological processes I'm most familiar with. If OP, or the putative soul-seller, is Catholic, then other rules may apply. I am more familiar with LDS / Mormons. Their philosophy regarding souls is quite close to the Baptists of my childhood, with the significant difference of the soul being redeemable after death (via baptism of the dead). But this is the only human intervention allowed for the ultimate fate of someone's soul, and even then the dead person must consent to conversion.

As far as other religions, I'll have to let Muslims and Hindus speak for themselves. Buddhism would also have non-transferrable souls. Though OP's friend may have incurred a karmic debt (to himself) by being willing to give up the core part of his existence in return for a half-eaten sandwich. Whether or not this would be enough to doom him to a million years in a Buddhist hell, or an unpleasant reincarnation, is not a question I could answer.