Ana y.

I booked the ticket in the morning and the price drops down at night. I called the agency and the airline, no refund.

Here is the policy for US passengers: https://www.going.com/guides/refund-for-fare-drop

If the fare drops within 24 hours of booking…
For all flights into or out of the United States, airlines are required to abide by what's known as the "24-hour rule" in the industry.

Under this US Department of Transportation policy, airlines must allow you to cancel or change the ticket within 24 hours of purchase without any fees (alternatively, they can allow you to lock in the price for 24 hours at no charge, but most opt for the 24-hour cancellation or both). The rule applies to any flight that originates or lands in the United States, is booked directly with the airline, and was booked at least a week in advance of departure.

In other words, if you discover a lower fare and you're within 24 hours of booking the first ticket:

Double-check that you're within the 24-hour window
Book the new flight immediately to make sure you get that lower fare
Cancel the original (higher-priced) ticket
Just make sure you book the new ticket before you cancel the old one. That way you won't be left with no ticket at all if the price rises again.

Note that some online travel agencies (OTAs) have their own version of the 24-hour rule; we've outlined the details of each in our article about the 24-hour rule, but be warned that these rules often come with a bunch of caveats. Read carefully.