How can one trust a fatwa that permits Islamic finance, if the same source of fatwa had previously permitted working with commercial banks?

Working in usurious commercial banks is forbidden because it directly involves usurious contracts or aids them. "Allah has cursed the one who consumes Riba (usury), the one who gives it, the one who records it, and the two witnesses to it," and He has prohibited cooperation in sin and aggression. However, working in financially independent branches that adhere to Sharia rulings is permissible, as in this case, it is as if it were an independent bank.