I read it carefully and understand the process, but disagree with what is recommended. The same can be accomplished in a simpler manner.
You have jacekm setup a base currency, enter foreign exchanges, set foreign currency for suppliers, and convert costs incurred in his country and paid for using local currency into Euro denomination. None of that is necessary to achieve what jacekm wanted or what siyab wants. Maybe I am wrong, but I will explain my logic so that you can point out where I err.
Point 1 is that incorrect wording is used when jakekm and siyab say that they paid for a purchase in USD or Euro. In fact both paid for their purchases in local currency. They do state clearly that they paid for the foreign purchases from a local currency bank account. The bank did send the money to their suppliers as foreign currency, but that is a service provided by the bank. I know it is common to say “I paid in Euros”, but it is not precise in these examples.
Point 2. Both jackem and siyab know all their costs in local currency. For the Suppliers that are out of country, the banks, when they post the transaction, post them in the local currency. (You do point out in the linked post that there may be a timing issue that would require an adjustment, but that is true no matter how the invoice is prepared).
Given those points, and in the context of producing a Purchase Invoice that lists all costs for an Inventory Item, the easiest method is to record everything on a Purchase Invoice that uses the base currency. Even you say at the beginning of the linked post, “Possibly the simplest way would be only to take up the transactions when the money leaves your bank account so all amounts are in ZAR”. That is all that I suggested to siyab (perhaps poorly). I stated, “Now you can put all the costs on the one invoice in AED.”
Here is a purchase invoice for siyab’s example using the method you recommend in the linked post:
And here is a purchase invoice as I suggested siyab make it. Notice there is no base currency so no “AED” all over the place, no exchange rates need to be entered into Manager, the Supplliers are not set to a currency, and the amounts on the purchase invoice are the same as would appear on the bank statement. This purchase invoice produces the same Inventory Item cost as the above, but is easier to do and understand. I think this is a preferrable way.