If I am ordering something for manufacturing based on my unique custom needs, like furniture for example, without providing material by my side, I want to know regarding payment options I have with me. Options like to pay full in advance or in parts or at the end.
I know that I can use bai-as-salam method to pay full in advance. And there's something like Istisna which allows me to pay in parts or at the end.
But I am concerned about proof validating the Istisna. I couldn't find reference in any hadees. People mentioned that prophet [saw] ordered for manufacturing of finger-ring and pulpit as proof for Istisna, and I found that hadees in Muslim and Bukhari, but payment method isn't mentioned in those hadees so it doesn't seem as a proof.
The other proof being mentioned is the ijma of scholars for Istisna to be correct, so I wanted to know the proof or reasoning which scholars have put forward for this, and preferably it should also address why the general rules which are being not followed here, like one must have possession before selling and there should not be debt from both the sides.