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.