The source of "cum-ex"?

"It May Be the Biggest Tax Heist Ever. And Europe Wants Justice." NYT 1/23/2020:

Martin Shields and Paul Mora met in 2004, at the London office of Merrill Lynch. […]

Today, the men stand accused of participating in what Le Monde has called "the robbery of the century," and what one academic declared "the biggest tax theft in the history of Europe." From 2006 to 2011, these two and hundreds of bankers, lawyers and investors made off with a staggering $60 billion, all of it siphoned from the state coffers of European countries.

The scheme was built around "cum-ex trading" (from the Latin for "with-without"): a monetary maneuver to avoid double taxation of investment profits that plays out like high finance's answer to a David Copperfield stage illusion. Through careful timing, and the coordination of a dozen different transactions, cum-ex trades produced two refunds for dividend tax paid on one basket of stocks.

One basket of stocks. Abracadabra. Two refunds.

You can learn more about this from the source at  cumex-files.com. But since this is Language Log rather than Evil Bankers Log, I'm going to focus on the claim that "cum-ex" is from the Latin for "with-without".

Because it isn't, really — and therefore I'm asking LLOG readers to help me figure out where the phrase cum-ex actually comes from. It's true that cum means "with" in Latin. But according to Lewis &  Short, ex basically "denotes out from the interior of a thing".

As often with basic spatio-temporal terms, ex can have spatial meanings ("In a downward direction, from, down from, from off", or "In an upward direction, from, above"); temporal meanings ("From a certain point of time, ie. immediately after, directly after", or "From and after a given time, from … onward"); and a bunch of extended meanings, "In other relations, and in gen. where a going out or forth, a coming or springing out of any thing is conceivable", such as "To indicate the material of which any thing is made or consists of", or "To indicate the cause of reason of any thing, from, through, by, by reason of, on account of", or "To indicate a transition, i.e. a change, alteration, from one state or condition to another", or "To designate the measure or rule, according to, after, in conformity with which any thing is done".

The Latin for "without" would be sine — but the scam was called "cum-ex", not "cum-sine".

No doubt "cum" and "ex" come from some fragments of legal Latin involved in the origins of the scam. But I'm drawing a blank about what the fragments were. And Wikipedia's List of Latin Legal Terms doesn't help me — here are all the entries containing either cum or ex:

compensatio lucri cum damno cum beneficio inventarii cum onere

ex aequo et bono ex ante ex cathedra ex concessis ex delicto ex demissione ex facie ex fida bona ex gratia ex officio ex parte ex post ex post facto ex proprio motu ex rei ex tunc ex nunc ex proprio motu ex propriis sensibus mora solvendi ex re mora solvendi ex personae ex intervalo temporis

The last one is suggestive — it's given in the definition of uno contextu as

Contemporaneously; when the phases of something are done without interruption or any intervening action; specifically, executed in one single execution ceremony (vs. ex intervalo temporis)

But then where's the cum?

Ideas?