If he sells the fence for 300 bitcoin valued at $100 each $3000
By the time he gets home they're $85
Loss of $450
Big deal, the fence is still worth 300 bitcoins. If you care about dollars, then price the fence in dollars and only take dollars as payment. Once you started pricing goods relative to multiple currencies at once, then you have lost the plot, as exchange rates vary all the time unless the currencies are pegged to each other.