Rules that apply for airlines are based on two things:
- The rules of the country under which the airline is registered.
- The rules of the relevant authority that has jurisdiction over the airspace in which it is operating.
In your specific question, you are asking about a foreign airline. These airlines are only bound by US rules when operating to/from the US.
Therefore, any DOT rules or other FAA regulations do not apply - in your specific case.
However, if a foreign airline is operating to/from the US, then it is bound by the rules enforced in that jurisdiction in addition to any rules that it is bound to based on the airline's home domicile.
A recent example of this was the case of Kuwait Airways, which was sued under US non-discrimination rules for refusing to selling a ticket to an Israeli citizen on their flight from JFK to London.
In that case, Kuwait Airways was bound by Kuwaiti Law which prohibits dealing with Israeli citizens or companies. The airline argued (unsuccessfully) that it was bound by its own rules and was not discriminating.
In the end, the airline decided to stop their JFK to London service as it could not circumvent its own country's laws and satisfy US regulations at the same time.
They still operate their JFK to Kuwait service (which is a non-stop flight).
You can read more about it at this website.
Specifically for your situation; most airline websites - and all travel agents allow you to hold a reservation for a certain time. That is, the seat is blocked for you, and you have to pay within a certain time period to keep the reservation.
I have often done this when applying for visas where you are not guaranteed an approval and may be on the hook for any cancellation charges if you book a flight.