Finance, Dune, RPGs… I think I found my specialist subject… 
First we need to separate out our 2 main sources: the Dune novel (mainly the Dining Hall scene) and the Dune Encyclopaedia (Non-Canon). The novel gives us very little, but we can extrapolate a few things. The Encyclopaedia has an entire entry on the Imperial Monetary System.
The novel introduces us to a representative of the Guild Bank in the Dining Hall scene. He is described as a Banker, the Financial Advisor to the Water Peddlers Union and Jessica also notes that he was a Harkonnen Agent and wonders if that means the Guild was against them.
From that we can see that the Guild Bank is considered to be closely connected to the Spacing Guild (more than just having the same name) but that it is not staffed by Guild members from Tupile, even at the highest level. (Can’t get much higher than Arrakis in the Guilds eyes)
That suggests (to me at least) that the Guild branches on each planet are staffed by locals while the Interplanetary aspect is handled by the Guild.
The Dune novels also talk about how a briefcase of Spice was able to buy half a planet, though this was in the chaos of the post Atreides period so the value may have risen.
The reason I mention it is that it suggests that for sufficiently large transactions Spice is considered a viable means of exchange.
It also has the advantage of being untraceable and compact, so may be a favoured means of exchange for smugglers and other less salubrious types who would want to avoid any Imperial Entanglements.
Bringing in the Dune Encyclopaedia we have a lot more information. Though again this is officially non-canon.
In this we hear about how the Guild specifically persuaded the Landsraad to set up an Imperial currency, the Solari. (AG 489 if you are interested) As part of this they became the effective bank to the Imperium. Each Heighliner held it’s own Purser who acted on behalf of the Guild to: collect & disburse, loan & borrow, hold in trust, extend & withdraw credit, cash drafts and make change. As each Heighliner meets up or returns to Tupile then ledgers are exchanged & updated with the latest records.
So immediately it seems that the Guild doesn’t seem to have banking facilities on planets, presumably this is where the Guild Bank Representative comes in. It is far easier for most of the planet bound people to deal with the Representative who then deals with the Purser on the Heighliner. Presumably there is some form of accreditation for those Representatives.
I see the Guild bank as less like a normal bank that you and I use and more like the old Venetian trading houses or even the Knights Templars. These provided banking drafts that you could take with you and cash in at another branch instead of carrying large amounts of cash everywhere. They would also loan money (at interest, not very Christian for the era) and provide other financing for exports and similar.
The other main aspect from the DE entry is that the Guild fixed prices for commodities for set periods (about 3 years). This removes a lot of the arbitration risk in trade. My 3 tonnes of Pundi Rice is the equivalent of 87 grams of Molybdeum or 1ml of Spice anywhere in the Imperium. Of course this only counts for goods being traded through the Guild system.
This next part is how I see this working in practice.
I would say that interplanetary banking works in one of 3 ways:
- You get a bankers draft from Representative/Guild Purser on planet A and then cash it with their equivalent on planet B.
- You place a request with the Representative/Guild Purser to pay X person on planet B.
- You have a standing arrangement with the Guild to be able to draw funds at any planet with the balance to be settled with funds deposited with the Representative on planet A.
I see this last one as basically how the House Majors work and any House Minor which has extensive interplanetary trade. It is the closest you get to having an interplanetary bank account.
It requires a lot of trust from the Guild so this sort of offer is only going to be extended to credit worthy candidates and probably generates a fair profit for the Guild.
For those who don’t have this facility they need to open a local account on each planet they work on and deposit a draft or arrange a loan. They can then use it as a normal account.
Most nobles will have some form of Solari cash they can just hand over or bankers drafts for the larger amounts.
Or as you said they will have people in their entourage to handle that sort of thing.
