How does a merchant ensure sufficient value in light of the price volatility? When a merchant accepts bitcoin payment (for example 10 btc for the purchase), the buyer send the payment of 10 btc and it is settled.

However, the 10 btc value will fluctuate every minute (up to 20% or more sometimes on a daily basis).  Let's say 1 btc = $2000 at the time of the payment and 1 btc = $1900 at the time when the merchant exchanged it for US dollars. The merchant lost $100 in this case and the opposite could happen.

How does a merchant ensure that the btc payment satisfies the cost of the product?

Please help me understand how this works. Otherwise, it does not make sense to me why Merchants need to accept btc payments.
There is no way for the merchant to ensure that he receives full value. This is the nature of digital currencies and why many merchants decide to not accept bitcoin as a payment; it is volatile. However, there is also the chance that the value will increase, thus netting a profit for the merchant. All in all, any merchant who accepts bitcoin not only is hoping for an increase in the price of bitcoin but is also looking to appeal to more customers.