An "I stole it" story is considered unreasonable and metagaming.
If it is unreasonable to use an "I stole it" story for your own benefit, then it is also unreasonable to force an "I stole it" story onto someone else for your benefit or their harm.
And that is exactly what this "exclusive contract" scenario does : it forces an "I stole it" story onto someone else in order to allow you to attack them. It takes a trader who did absolutely nothing wrong and paid his own hard earned credits to purchase a cargo load of a commodity, and accuses the trader of having "stolen" it. Metagaming to harm someone else is just as wrong as metagaming to benefit yourself.
This accusation is paradoxical to begin with. If you-in-space know it's stolen, then the umpteen guys on the base who loaded the cargo would have known the same thing; stolen cargo right out of a production location is absurd because nobody can dock and get the cargo loaded without the proper paperwork/contract/etc. To think that mere possession indicates theft creates a paradox : the people on the base are just as likely to know this as some pilot in space is, in fact the people on the base are MORE likely to know than some random pilot in space is. And if the people who are responsible for guarding and loading the commodity on the base can't spot a thief, then it is completely absurd to think that some know-nothing pilot flying around can spot a thief based on EXACTLY the same evidence that already convinced the base employees that he was not a thief : mere possession of the cargo. Forged documents and fake identities are irrelevant as excuses to spot a thief, because the only actual evidence being used in this scenario is the combination of ID and cargo. If a pilot can spot a thief that way, then so can the base, and if the base cannot (which it does not) then neither can a random pilot. And it's just metagaming to claim otherwise.