I am not sure why Oyster▸ PAYG▸ has to deduct a high fare and refund it anyway. Even Oyster allows this to go below zero, requiring only 1.90 I think to let you through a barrier.
If you have not got enough on your card then either you should have a credit allowance (personal to your account) or you should have to pay the excess to a ticket inspector or at the barrier.
Simple
^1.90 being the minimum fare charge (a Zone 1 ride) and of course if you then travel to Zone 6, then it creates a negative balance which is taken when you next top-up.