Praise be to Allaah.
The rule that all things are allowed is considered to be one of the major well-known principles of Islamic jurisprudence (fiqh), and there springs from this principle the idea that all actions are allowed except those for which there is evidence to show that they are forbidden. There are some exceptions to this rule: in matters of sexual relationships, forms of worship, and eating meat, the rule is that it is forbidden except that which is expressly permitted; and it is forbidden to dispose of another person’s property except with his permission. Modern forms of contracts are permitted so long as they do not involve buying something sight unseen, deception, riba (interest or usury), fraud, cheating or other things that are forbidden by sharee’ah. And Allaah knows best.