ER Schema - bank system
The way I would initially think of this (as an accountant in a previous life) is not as a transfer, but as what is going on in MY bank's account? One stored procedure would deduct funds from one of my member's accounts, and a second stored procedure (initiated by the application) would send something to the other bank system to deposit funds. The record of the transfer would go into the individual account's records as just another transaction, like a deposit or withdrawal. There would be no lasting connection between the two accounts.
Even if the transfer was between a member's checking and savings accounts, they would still be treated as essentially two transactions going on at nearly the same time. One deposit, one withdrawal.