. . . No. This is just explicitly wrong. One of their sons was already transformed into a Chain Devil. The book doesn't care if you believe it or not, because it's quite clear that the parents actually sold the souls of their children to Asmodeus. Sure, it might not make sense or be internally consistent with 5e's lore, but in the adventure, the Cassalanters definitely succeeded in selling the souls of their children.
Which is why, IMHO, it's another reason for it being a bad adventure... As for the elder son, I think he was old enough to sell his own souls and got transformed, probably as an incentive for the parents.
After that, although it's not canon in 5e, I've always been fan of the old "trap the soul" spell, it was a great plot device for many reasons, including preventing raising the victim from the dead. I think that, because of the cruelty of the fate but also the consequences in terms of cosmology and the associated complications, it has not been continued in the game, but note that you can still do very nasty and complicated things using magic jar, especially with multiple castings (trap someone else's soul in a container, then cast it again for example so that you have another container to go to, then possess / return to your own body, note that the spell is not completely clear and precise). You might then have containers with someone else's soul in there, that you could trade off, possibly to devils. It does not mean that the creature is damned, it is still linked to the fate of its soul depending on its beliefs and alignment, but you might trade if away.
But still, see below, as per the soul coin description, they contain evil souls, although you can of course make exceptions.
(Furthermore, in BG
iA, Thavius Kreeg manages to sell the souls of everyone in Elturel to Zariel.)
It's not the way I understand it, people signed their soul over to Elturel via the Creed Resolute, but the fine print then redirected this to Zariel. Note that this leaves the case of the aforementioned Halfling, which I think only works if the halfling in question has not signed the Creed Resolute - which is certainly a possibility.
In the end, I don't think I've ever said that all the souls in the coins were evil, I just think most of them are, as they are forged by Mammon who sort of controls the soul trade ("As the lord of Minauros, Mammon oversees the soul trade"). When you combine this with the sentence at the end of the Soul Coin description ("Soul coins are created by Mammon and his greater devils on Minauros, the third layer of the Nine Hells, in a vast chamber where the
captured souls of evil mortals are bound into the coins. ") you clearly see that most of the Coins, if not all, contain evil souls.
Again, it does not invalidate plot devices as above with Trap the Soul, but it still means that having your soul capture in a coin and used at the very least as currency, and as a prison where your essence can be drained by any devil, or consumed one way or another, is not unjust punishment.
