Buer is grimoirically associated with the Sun being in Saggitarius. If you were to think of this in natal astrological terms, Buer would be a Sagittarius so that's why his dominion falls there. Spirits have astrological generation and the secret lies within their names. The letters in their name outline a natal chart thats tied to when they were generated.
Now his Mercurial association comes from the powers he rules over, which would be plants (medicine), philosophy, and logic. These are all classically Mercurial and Thoth attributed qualities. Now within the hermetic philosophy, Mercury is closely tied to the Moon. They are coparticipants. Within the Kore Kosmou Isis attributes her magic and initiation to Thoth, saying that he was the one who initiated her and taught her magic. Isis's Lunar qualities are attributed to Mercury in this passage. When we dig into the ancient Egyptian religion before hellenization (Isis becoming a moon goddess and Thoth associated with Mercury) Thoth was originally a lunar god. If we look beyond a surface level understanding of this being an accident of syncretism and take it from the perspective of being a process of esoteric Divine inspiration then we can reconcile Thoth and Isis, or Mercury and Luna, being closely intertwined and coparticipators.
If we think about the function of the Moon and Mercury, the Moon acts as a gateway between the Earth and the Heavens, while Mercury acts as a gateway between God and the Cosmos. They both participate in this gateway phenomenon where Mercury hands down the Divine to the Cosmos and Luna hands down the secrets of the Cosmos to Earth.
In astrology Mercury is the only planet that is both male and female, benefic and malefic, completely dependent on what he is aspecting to choose what nature he expresses. Whereas all planets are dependent on Luna's placement, Luna is the most important planet in the sky in terms of magic and it is through her ALL things pass. When we select elections for magic we must first make sure the moon is in good aspect and dignified and then we can begin to approach the other planets. So astrologically speaking both of these planets are once again showing a sympathy with each other in their nature.
The suggestion of the Moon in Saggitarius has to do with operative conjuration procedures as all things pass through the Moon and she governs the gateway it is better to conjure a spirit when the moon is in its sign. She moves faster, is more dramatic, and provides access to liminal spaces.
What you can extrapolate from all this information is a few things about Buer. His ruling planet is Jupiter as that governs his Sun sign Sagittarius and his powers fall under the auspices of Mercury. So what I would do with this information is conjure him with the Moon (she provides access to spirits) in Sagittarius and aspect Mercury either in conjunction, trine, or sextile with the moon. Bonus points if Mercury is in Saggitarius conjunct with Luna. And then even more points if you can swing Jupiter in good aspect on the chart as well.
The easiest method though, would be following the placement of the Moon in Sagittarius. Demons are powerful and immediate entities and don't require astrological intricacies to get immediate and noticeable effects, but at the same time the more attention you pay to the placement of the planets you can exponentially increase the power of your spell. And Buer would probably be incredibly pleased with that much attention to detail.
As for the cancer thing, the moon is exalted in Cancer, so conjuring Buer in Cancer would have to lean on the authority of Eladiah as the intermediary. At that point your not really working with Buer but using an auspicious time to use Eladiah to command Buer for you.
