Hello

i have a question

I had to cancel a flight before departure due to illness they told me that the ticket is not refundable but i have a flight credit so what i want is to use the same route but in a different date, i called them and they told me the price of how much is going to be(with fees and everything) when i asked to the representative if i can pay over the phone with my card she told me yes and that once she collects the amount i have to wait 24 hours to see the confirmation in "My trips" because they have to make sure that they have availability, my question is what would happend if they collect the amount and realize that there is no availability for the routes that i mentioned to them , will they refund to me the money?
will they collect more money?

tha airline that i booked throught them was with Lufthansa/United.