Allah commanded us with things, and we must comply with what He commanded. Every accountable Muslim has the obligation not to engage in anything until he knows what Allah ordained as lawful or unlawful about it. Allah made the selling (bay^) lawful and the usurious gain (riba) unlawful. The Islamic law (Shar^) defined the selling with the definite article to indicate the lawfulness of only the selling which satisfied the integrals and conditions and not any selling per se. One must learn the aforementioned if one wants to sell or buy or else one will consume the usurious gain (riba), whether or not one wanted to do so.
The Messenger of Allah, sallallahu ^alayhi wa sallam, said: <<The truthful tradesperson shall be assembled on the Resurrection Day with the prophets, the highly-ranked righteous Muslims (siddiqun), and the martyrs.>> This is because of what this person faces when struggling with himself and his desires, in subduing his self to conduct contracts according to the Islamic law. The threat of Allah to punish those who violate His Laws is known.
Among the remaining contracts are: the renting of things and hiring people's services (ijarah), trading with another person's money for profit sharing (qirad), putting up collateral (rahn), commissioning another to do something (wakalah), deposits for safekeeping (wadi^ah), lending something for use (^ariyyah), partnership (sharikah), and tending (watering, weeding, etc.) grapes or dates for part of the crop (musaqah). One must also observe their conditions and integrals.