In certain lore it is believed that humans are the only intelligent being in all creation to possess a soul.

Demons and Devils collect souls, but they can't just take yours. You have to agree to it. There are checks and balances in place to keep them from simply taking it. Else, they just would.

There are different ways they might go about it. Some use greed, simply offering you your grandest desire for your soul. Some use greed, simply offering you your grandest desire for your soul; great wealth, power, great guitar skills ( look up Blues Guitarist Robert LeRoy Johnson), etc. In OP'S brother's case, a better childhood for he and his brother.

Trickery, deceit, threats, direct violence and torture are all possible, but unless you willing give up your soul, no such creature can take it.

These beings make amazing lawyers and there's no such thing as Spirit of the Law in these cases, but they still must abide by the rules - they can omit certain details and use confusing jargon, but one thing they can do on a contract is lie. Also, they can't just cut you and have you bleed on a blank contract, or else they'd just do that every time.

There are certain conditions that must be met - the agreed upon result; money, power, skill, nice childhood, etc., must happen, as worded. Also, a person must be of proper age - having gone through puberty- and understand the contract, or at least, agree to the wording of said contract. In fact, in the cases ive studied, the contract must be read aloud, by either the soulee (coined term), the being presenting the contract (hereafter referred to as Demon), or a mutually agreed upon third party, in a language he understands. Including asking if he understands what he's signing and answering any questions.

While OP is in some degree of danger, his soul is still his own. (Although, he doesn't seem to be aware of this and the demon is not required to inform him. If he simply accepts his "fate" and follows the demon, then that would probably count as Willing.) The demon has no case.