Matches two bank transactions as a transfer. The source transaction may be incoming or outgoing; the
transaction amounts determine the direction. Cross-subsidiary transfers require access to both subsidiaries.
For a same-subsidiary transfer with a difference, provide reconciliation_adjustment.account_id;
reconciliation_adjustment.fields apply to the adjustment line. If either bank transaction has a fee, provide
fees_account_id to book it. A transfer with fees on both bank transactions is not supported.