The branch of Islamic law that covers financial and commercial dealings, such as trade, debt, employment, and the handling of money within a family, is called mu'amalat.

Islam treats honest trade as a legitimate and encouraged livelihood, in clear contrast to interest. In Surah Al-Baqarah, chapter two, verse two hundred and seventy-five, an English translation of the meaning reads: “Allah has permitted trade and has forbidden interest.”

A valid sale rests on the mutual, informed consent of both parties. In Surah An-Nisa, chapter four, verse twenty-nine: “O you who have believed, do not consume one another's wealth unjustly but only in lawful business by mutual consent.”

Beyond consent, a few general conditions run through the classical treatment of sales. The item being sold should be something that can be lawfully owned and delivered. Its price and description should be reasonably clear to both sides. And neither party should be deceived about what they are getting.

One specific thing that is prohibited is gharar: excessive, unnecessary uncertainty about the subject or outcome of a transaction. A hadith in Sahih Muslim, number fifteen hundred and thirteen, reports that the Prophet “forbade a transaction determined by throwing stones, and the type which involves some uncertainty”. It is an early example of selling something whose outcome was left to chance, rather than clearly agreed.

These principles, honest trade, mutual consent, no interest, and no excessive uncertainty or deception, are the shared foundation on which scholars across the four Sunni schools build. Where they differ is often in how the principles apply to particular structures: exactly what counts as interest in a given arrangement, or how much uncertainty is tolerable in a given kind of contract.

This lesson is a general introduction, not a guide to structuring any particular transaction. Modern financial products, such as mortgages, insurance and investment funds, raise detailed questions, and qualified scholars and Islamic finance bodies can reach different conclusions about the same product. For anything beyond a simple, everyday purchase, seek guidance from a qualified scholar or a recognised Islamic finance authority.