If I build a match with price adjustments (member discount for example) and a person selected that discounted rate and prepays for the match and is then queued for approval, can I adjust the amount due if that person is not actually a member or does it need to be resolved external to PS?
You can do a few things.
You can withdraw, refund, then delete them and have them reregister at the correct level.
Or
You could refund them, remove the payment then submit to them a custom price of the correct amount. Except when you do it this way it is incumbent on you to watch over it to see if you get paid.
Personally I would choose option 1 and then I don’t have anymore admin things to do when they re register. Unless they try and register as a member again… 
Regards,
D.J.