In what way are you swindling them?
At some point, fifty years from now, maybe two hundred years, their technology will advance to the point where they need that unobtanium to advance further. We stole their supply with the equivalent of pretty beads.
Right, but they got a hundreds of years jump in other sorts of technology, so it’s a tradeoff, not a swindle. Your replicators and medicine and all that would’ve improved billions of lives by the time they figure out what they’re missing, and it’s not clear from the hypothetical that this missing substance is totally necessary rather than nice to have. Like of aliens came by 500 years ago and traded modern medicine and power generation for all the aluminum on our surface - we’ll miss that aluminum but we’ll work around it and the uplift they gave us would’ve been worth it.
If it makes you feel less of a pirate, offer them full Federation membership in exchange for the unobtanium. That way they get all the tech, and don’t have to worry about losing the unobtanium because as a full member, they have the same access to the stuff as every other member.
Point this out to them in private, then secure an obscene kickback for telling them and to keep it on the QT. Then you retire to Risa.
If you don’t, they may elect to join the Klingon Empire instead. They may be second-class citizens (not sure how racist Klingons are supposed to be), but if they have honor they might at least avoid being enslaved and they would have access to advanced tech (probably more weapons than medical, in this case) and markets.
Who’s racist?:
Stranger
You’re assuming something beyond the hypothetical. Trek tech isn’t usually depicted as so linear e.g. there are multiple weapons types, multiple ways to travel FTL, etc. etc.
If the spacefarers only traded, say, pretty holograms for the mineral, you’d have a point. But I, and others, have been talking about trading substantial tech.
In any case, where in the hypothetical does it say we’re stripping their planet of all of this mineral, anyway?