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Can we amend last year's Schedule C to indicate reduced income due to a customer refund this year for a product we sold last year?
I am not an accountant, but I have a light accounting background, despite being primarily an engineer.  I also have a tiny schedule C business which has both better and worse years.  I am also in the United States and pay US taxes.  I assume you are referring to the US Form 1040 tax return, with the attached Schedule C.  However little I know about US taxes, I know nothing about foreign taxes. You are a cash-basis taxpayer, so the transactions that happen in each tax year are based on the cash paid and cash received in that year.  You were paid last year, you computed your schedule C based on last year's actual transactions, and you paid taxes on that income. You can not recompute last years schedule C based on the warranty claim. You might want to switch to an accrual accounting method, where you can book allowances for warranty claims.  It is more complex, and if your business is spotty and low volume, it may be more trouble than it is worth. At this point, you have two months to look for ways to shift expenses into next year or being income into this year, both of which help offset this loss. Perhaps a really aggressive accountant would advise otherwise (and remember, I am not an accountant), but I would take the lumps and move on. This article on LegalZoom (link here) discusses how to apply a significant net operating loss (NOL) in this year to the previous two years, and potentially carry it forward to the next two years.  This does involve filing amended returns for the prior two years, showing this year's NOL.  For this to be relevant, your schedule C loss this year must exceed your other W2 and self-employment income this year, with other tests also applied. Perhaps a really aggressive accountant would advise otherwise (and remember, I am not an accountant), but I would take the lumps and move on.