Let me put it another way.
Suppose you booked a trip for someone that was $1000, of which $100 was your commission. They cancel and you refund the $1000. You can report the $1000 received as gross sales and the $1000 paid back as an expense. You don't record anything extra for the lost $100 of commission income. You never received it, so it won't be taxed, but it's not an additional deduction.
