The best thing for the doctor is to get paid (and pay taxes on) $400. The $300 deduction is of no value to them because they never recognized the $300 as income. The deduction is basically "Hey, sorry you couldn't collect your billed amount. You don't need to pay taxes on money you didn't earn".
Sorry if you fully understand this already. The lack of understanding of basic taxes is a pet peeve of mine. People make statements like "Johnny was looking for one more tax deduction, so he donated to my charity". Johnny probably cares about the charity or at least looking good in the community, because he'd have a larger net worth if he just sucked it up and paid taxes rather than donating it.