EBP - Backend Reservation - Availability Check

Hello

We have a requirement for doing classic scenario with reservation if for inventory items. The system (SRM 4.0) is configured to determine whether to create a reservation (if stock available) or external procurement in the backend R/3. The standard function checks for sotck availability at the time of shopping cart creation. The question is if the system can check the stock availability right after the final approval. Is there a BADi I can utilize?

Another question is regarding to services procurement. The standard EBP supports buying services by quantity (based on UOM) and with value. Can the standard EBP procure services by value($) only? For example, if the buyer has a fixed amount budget to procure services, he/she does not want to specify days/months for the services but the budget amount. How would the system support this?

Also, the same question on the receiving/confirming end (receiving by value).

Your comment is much appreciated.