I don't really get what you are trying to point out here.
The dollar's value in other currencies, commodities and Bitcoin varies all the time. You will need a time stamp of the exchange rate at the time of buying Bitcoin and the time of selling bitcoin.

The difference in dollar value will be taxable.

You can argue the semantics of the USD and if it has any value at all, but the IRS won't care. If you feel that it is worth arguing, I would suggest that you don't bring it up with them personally, unless they audit you!
Otherwise I think you might find that they suddenly want to audit you, and may not be too interested in your definition of the USDs value!