Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
That is the risk in any business. The seller should know about that. But if he is okay with that, I think he can hold his bitcoin for some time until the price increase high so he can sell his bitcoin, and take another profit. But not all sellers will okay to see the price changes every day, and maybe some of them will shock to see bitcoin price can increase and decrease to any price in a second.