Vendors - whether original makers or resellers - are free to offer differential pricing to customers, as long as they don't break equality laws.
Only Steinberg/Dorico are making a sale, I don't see any reason to think Finale are forcing their hand on price.
More likely it's Finale says "offer a good price to our users, and in return we'll promote it to our user-base", and Steinberg decide what that price will be.
Confirmation that the license exists has to be handled a bit carefully for GDPR reasons, but that's not insurmountable. Competitor cross-grades are pretty common practice in those (fewer and fewer each year) sectors which still sell perpetual software licenses.
As @Earw0rm mentioned, this isn’t a “resale” in the sense that that word is defined in the law. Buying Dorico at a discount is not a “resale” of Finale, regardless of the agreement; they’re different products sold by different companies. There is also no evidence, no reason to believe that Finale had any say in the sale price of Dorico. But, yes, the companies did agree to nudge customers from Finale towards Dorico.
The nudge is not mandatory, and the transition is opt-in and not automatic, so customers have the choice, and that lessens the probability that this crosses any laws. Dorico seems to have healthy competition from Sibelius, MuseScore and others, and the market has been called “saturated”, so no clear indication the result would be a monopoly. We also don’t know if Finale solicited agreements from other competitors who declined to participate, and if so that also reduces the possibility of this appearing shady in any way.
This kind of thing happens all the time when products and companies merge or die. It’s legal and acceptable for small companies to merge, and becomes illegal when merger gives the target company too much power, where ‘too much’ is determined by the court. In this case, since these are relatively niche (small) products, and it’s not a corporate merger but simply a targeted advertising & discount sale to a captive audience, and since the result probably isn’t a monopoly for Steinberg, it’s unlikely that any court would care about this agreement. Small companies are allowed to make agreements and do all kinds of things that large companies might get in trouble for.