I believe it can be spelled either way. At least according to J.M. Hamade, a scholar on the subject. I have heard that they are tricksters and that it is very important to set clear intentions and boundaries. I very much appreciate your solid advice.
There is a reason for the D.. But you choose as you will.. It can be both ways, indeed. I advise against haste. And careful contemplation of outcomes via a methodology which you can, if worthy, develop to establish a pristine wish. Good luck.
Since you are polite and learned, which I am unused to meeting in my travels, I will give you another wish asset. Incorporate the requirement for djinn to adhere to the motto and its actual meaning at the highest levels.. 'Ethics.. Sapiential, Veritas'.