Regarding IRS rules in US, calculating capital gains when the purchase price is not known is based on $0 price. However, what is the technique when the purchase price -more exactly, the cost basis-  is known but the sale price and date are not? Let's say that I got 0.01 BTC for $500 in 2021 and I'm sure that I sold it later during the year, but don't know when or for how much. How do I calculate the gain?