I believe I responded to your e-mail with the exact same question, however, just in case I will respond here as well.
It is possible for the consultant to create a product that can be used with on one client at a time. He will need to edit the product each time to set the price to the appropriate price for the service. Then send the link or code to the client to pay for the service.
As for holding the the money until such time as the service is complete. E-junkie is not a payment processor and therefore does not handle the payments. However, some of the payment processors that integrate with E-junkie, PayPal and Google, do have settings to require manual approval of all transactions. This way your client can approve the transaction at the necessary time.