After you click "Propose" on a Buyer, your proposal will be available for the Buyer to see in their Buyer Account, but they will not get an email notification until the next day.
During this time the credits will be deducted from your Available Credits balance and held. You can still "Undo" the proposal if the Buyer hasn't viewed it and get your credits back.
Once the Buyer views your presentation, the held Credits will be deducted permanently. However, if the Buyer does not open your proposal after 14 days, those credits will be returned to you.