“He created man (Adam) from sounding clay like the clay of pottery. And the jinns did He create from a smokeless flame of fire.”
The Noble Qur'an - Ar-Rahmaan 55:15, 15
When Allah made Man he also made the Djinn. Since that time the Djinn have established a set of courts that each relates to. Now each is a member of one of seven courts and all except one of those is the combination of the two elements from which Allah made the universe.
The power of Invoker is not her own but bargained for with the members of these Courts. The Invoker may be a friend of Djinn, and favoured by the Court, but ultimately gifts must be given and favours exchanged. The more powerful Djinn will only bargain with those with an established reputation at their Court and the greater the service to be delivered the greater the payment required. Till her name if known the Invoker will only be able to deal with Imps and the least Djinn.
As a creation of Allah and intelligent in a way similar to Man, the Djinn have free-will. And since they have both mind and will they may receive the word of Allah in the same manner as Men and may listen to His Qu'ran and be saved.
But Allah commanded the Djinn to bow to Man and some refused. These rebels are not all evil, some are merely wilful and capricious, but the greatest of all Djinn - named Iblis, but now Shai'tan - founded his own Court and schemes blackly, though his bequests may be prodigious indeed.
The mage has learnt to communicate with the Spirits. At this level of power and respect, he is only able to impress and command the Imps and Lowest of Djinn - the impolite, demanding, mischevious children of The Court. Even at this level, the Djinn are able to perform such feats as finding treasure or causing a small storm, depending on the limitations of The Court.
Of course, all these wonderful feats require offerings to the spirit - the bigger the wish, the bigger the sacrifice. Perhaps a hundred fish for a water spirit, the gathering of 100 gold for an Earth creature, or the burning of a tree for a fire spirit. Spirits prefer these before hand, but sometimes offer wishes without mentioning their demands if they're especially spiteful.
With this boon granted a mage has personal access to one of the powers of a Spirit of one of the Courts. A Djinn of the Simoon might grant the Invoker the ability to produce a dozen dust-devils to sting and distract and annoy, an Imp of Brass could permit a mage to bend and twist metals.
Djinn are loathe to grant such powers however; the Invoker may act without their knowledge and bring upon them trouble from those more senior in the Court, and the Spirit loses some of its own power. For this reason the instant-use of this Spirit's power may incur a regular fee to be paid, or perhaps a single large initial payment. The Invoker also risks the Djinn's wrath should it feel its powers have been abused.