"I'm asking you to middle a diamond"

In the movie Donnie Brasco, Al Pacino shows a jeweler a diamond and tells him

"I'm asking you to middle a diamond for me here."

What does it mean "to middle"?

I cannot found 'middle' as a verb in any English dictionary-only as an adjective or noun.

Solution 1:

Surprisingly, middle (verb) is not found in Lexico, or in Merriam Webster. Those two references usually cover "across the pond" differences in semantics.

However..

a middleman (noun) is...

an intermediary or agent between two parties especially : a dealer, agent, or company intermediate between the producer of goods and the retailer or consumer

-Merriam-Webster

The slang usage has evolved as a form of verbification, and is understood in certain quarters to mean 'act as a middleman'. I have heard the expression used, and have used it for over 40 years, in various business environments. In my opinion, it is quite established in American English.

I cannot speak for BrE. usage.