If someone were to offer a service that holds transactions in escrow against a pending bitcoin transfer, receives the money into a holding account, and pays an agreed upon dollar amount to the vendor and release the transaction.
That someone would have just made a very cheap bitcoin purchase*. You can then apply traditional foreign exchange arbitrage strategies to determine when to sell into a hard currency and how. Refunds to bitcoin could also be handled out of the arbitrage account which allows you to exchange out bitcoins that have fallen in value for currency you would be owed the merchant.
*Note: If you're charging a premium to the merchant, the cost of the exchange is actually negative to you!
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