In fact you may be understating it a bit. If the doctor uses cash accounting, then it's not that the $300 deduction is of no value, there is no $300 tax deduction.
As you point out, a cash basis taxpayer pays taxes on actual income received. Money you may have hoped to receive but didn't isn't income, so there is no tax and no deduction related to it.
If the doctor filed taxes using the accrual accounting method it would be different.
As I mention to a number of other comments...it simply depends on the accounting method.
It can be waived (not treated as income at all as you say) or it can be treated as income (taxes paid) and the loss carried forward for future deduction.
As you point out, a cash basis taxpayer pays taxes on actual income received. Money you may have hoped to receive but didn't isn't income, so there is no tax and no deduction related to it.
If the doctor filed taxes using the accrual accounting method it would be different.