When working with local non-profit organizations who have cause we agree with, we often underquote to fit in the project's budget. So if we are doing $2500 project and the budget they have is only $2000 , we will make an invoice for $2500 and deduct $500 as in-kind contribution from us. Only one who can claim these $500 for the taxes is us.
In your case, I would write the invoice for the full amount, and right on the invoice mark the difference as the discount. In no case can your buddy claim full amount on his taxes, but you could claim it yourself as things like that are just part of doing the business.
Personally if I was doing it for that price I wouldn't be supplying an invoice it would have to be cash in hand. If pressed I may consider giving an invoice but definately only for the actual amount paid. Perhaps it works diferently here but if my books show an amount then I have to account for that amount. I would considered if a person asked for a receipt for a larger amount they were being greedy.
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