Today, I found a letter written to the journal Nature from 1896 wherein the author of the letter (Kumagusu Minakata) shares a passage from an Arabian text written in the ninth century by a sailor and adventurer Sulaiman al-Tajir. The excerpt is as follows (it comes from an 1845 French translation of that document of which I used Google Translate to convert to English so it's not perfect):
Did the Chinese understand here that a fingerprint was truly unique and could be "matched" to a particular person if they reneged on the debt?The Chinese respect justice in their transactions and in court documents. If a man lends a sum money to someone, he writes a post about it; the borrower, in turn, writes a note, which he marks with two of his joined fingers, the middle finger and the index finger. 2 We put together the two tickets. We fold them one with the other, we write some characters on the place that separates them, then we unfolded and we give the lender the note by which the borrower recognized his ditte. If, later, the borrower denies his debt, they say to him: 'Bring the lender's note.' If the borrower pretends to have no ticket, that he denies having written a ticket accompanied by his signature and his brand, and that his ticket perished, we say to the borrower who denies the litigation: 'Declare by writes that this statement does not concern you; but, if, for its part, the creditor comes to prove what you deny, you will receive twenty sticks on the back, and will pay a fine of twenty thousand (fakkoudj) key brass pieces.
This person was alive during the 800s AD and is describing the purpose of the print, not just its placement on a contract.
As I keep digging, I'll keep posting what I find.
The author originally wrote a letter in 1894 and was unable to provide sources for some of his comments - I'm still trying to track those down but the original letter is below.