There is already a cap on student loan repayments based on discretionary income. The Reconciliation Act just lowers the cap from 15% to 10%.
“Discretionary income” is defined as adjusted gross income minus 150% of the poverty level (based on family size). If your hypothetical doctor did what you suggest, his consulting fees would be part of his adjusted gross income, just as it would be for tax purposes, and he wouldn’t reduce his loan payments by a cent.
I should add that “discretionary income” is just a phrase Obama used. It doesn’t appear in the language of the Reconciliation Act on student loans or in the existing statute it amends.
Thanks for the clarification.