You wouldn't do the bitcoin transfer until the transaction authorizing it completed successfully. And you don't just use a single transaction that deducts the a

You wouldn't do the bitcoin transfer until the transaction authorizing it completed successfully. And you don't just use a single transaction that deducts the amount. You'd use one transaction to indicate you were beginning a transfer, putting that amount in a hold state, attempt the bitcoin transfer, then a transaction to indicate the bitcoin transfer was successful or not. If there is an error in the bitcoin transfer or after, and the second transaction is never run, the funds are still in hold whether transferred or not and can be manually corrected by checking the system logs.