you simple create two records linked by one 'transaction', the source in both cases is the same account, while the destination for one of those postings is the fee account and the other destination is a merchant or similar account. And you can link as many of those postings under a single transaction
okay so now you’ve got double-entry bookkeeping except all of your credits/debits have two dollar values instead of one. let’s call it “quadrupal-entry”
yes, it's a form of double-entry bookkeeping - that's the base. If it weren't double-entry then indeed it would be a pretty poor choice. This design enforces double-entry at a fundamental level, it's never possible to create a record that doesn't affect 2 accounts - and that's actually the whole point of it :)
You have a single USD (or other) value - so in the simplest form it just looks like this:
From: Alice
To: Bob
Amount: 10
Currency: USD
And the balances are simply sum(transactions where account is receiver) - sum(transactions where account is sender)
