What should we do when scholars differ on a specific fatwa, such as the issue of depositing money in banks, and are we sinful if we follow the opinion of a scholar who turns out to have misguided us?

If scholars differ on an issue, it is not sufficient to simply adopt any madhhab; rather, one must examine the opinions and attempt to determine the strongest, based on the Prophet, peace and blessings be upon him, who said: "Consult your heart. Righteousness is that with which the soul is at peace and the heart is tranquil. And sin is that which wavers in the soul and causes disquiet in the chest, even if people give you a ruling and give you a ruling." So, if one's soul is not at peace with the fatwa of a mufti, he should ask another until tranquility is achieved. And he must fear Allah to the best of his ability.