All accounting journals are immutable in principle. That's why you can get a card refund instead of a voided transaction and consider yourself even with the store. Typically you'd want to preserve independence between time of entry and time of transaction though, so this is a lot like treating the journal as mutable from the account balance history point of view. Of course addition is commutative so if all you care about is real-time balance or inventory quantity then the order of journal operations don't matter and you don't have to rebuild account history for each new out-of-order journal entry. With crypto it's desirable to always present the immutable view so it's actually simpler than a general purpose accounting system i.e. no voided transactions and no rebuilding account history if a journal entry comes in out of order.