Sometimes these are legal requirements one way or the other. In my country, invoices in other currencies are based on the ECB rate of the date of the invoice, no matter what the actual value is when converted.
That doesn’t contradict the advice; the discussion above is about the accounting entries, not the invoice. An invoice issue is a commercial event, and the amount of the invoice is the actual value of the invoice, and will be the amount debited to accounts receivable and credited to income when the accounting entries are written, and must be in the reporting currency of that entity, so a conversion does occur on that date of issue.
If a different amount is accepted later in payment, then that difference is entered as an adjustment to income, probably coded to an account with a name similar to “gains/losses due to currency fluctuations”.
Do not confuse the commercial document/event for the accounting entries. The latter is a product of the former and they explain each other. In principle the ledgers can be entirely reconstructed from the primary records of commercial activity. The distinction is made even more clear by the existence of processes that validate one from the other viz. reconciliation and audit.
The broader takeaway is that your database of commercial activity must not use the accounting entries as its primary record. In my view the only data structure these two share is the chart of accounts.
Sometimes these are legal requirements one way or the other. In my country, invoices in other currencies are based on the ECB rate of the date of the invoice, no matter what the actual value is when converted.