Broadly, my question is: Is it illegal (on a scale from 'dodgy' to 'federal prison') to back-date an invoice?
Specific, hypothetical scenario:
Say you have an item for sale on eBay, and your auction ends on July 2. Some bids have already come in, and someone asks you to add a 'buy it now' price, because they want to make a purchase before the end of the tax year (why didn't I think of that?). You can't add a 'buy it now' price to an item that already has bids. So, do you offer to back-date an invoice for this person if they win the auction? If you did this, would it be illegal? People have offered to back date receipts for me quite openly in the past - but that is not proof to me that it is legal ;)
Cancel the listing and re-list with the buy it now price. Then you're a-o-k.