Unfortunately, if you are on the cash basis of accounting, which most small businesses are, whereby you recognize income as received, and expenses as paid, the IRS does not allow a deduction or tax benefit for income not received.
If you were $80 for a massage you normally charge $100 for, you would simply put the amount you were paid ($80).
If you used another method of accounting and claimed $100 of the income, you could at that point write-off the $20, but you're still left with a net $80 of income.
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