Just to clarify, there is no actual pending withdrawal status with Avios balances. They're either in the account or out of them. I agree it would be nice for transactions to show as pending, but there isn't any hold placed on them in the time between balance verification and withdrawal. It's possible to make another booking in that time (especially when there's a delay), but trying to overdraw your Avios balance will eventually cause one booking or another to be cancelled.