This article examines how non-codified Sharīʿah governing Islamic banking and finance agreements should be applied to ensure certainty and predictability of the applicable rulings. The significance of this topic stems from the fact that the multiplicity of schools of Islamic law or fiqh has given rise to concerns about the certainty of the applicable rules. Here we set out these concerns through the lens of English courts and argue that non-codified Sharīʿah has the status of a law in Jordan regulating Islamic banking and finance agreements. It overrides legislation and excludes Statute Law that could invalidate agreements acceptable in Sharīʿah. Further, the concepts of maṣlahah and istiḥsān are explained as bases for the selection of applicable Sharīʿah rulings. This approach ensures certainty and is better than codifying rigid rules from Sharīʿah that could impede the development of Islamic banking and finance.