The scholars (may Allaah be pleased with them) stated that zakaat al-fitr is connected to numbers of people, not money, and it is to be paid in the place where a person is on the night before Eid.
Ibn Qudaamah said in al-Mughni (4/134):
As for zakaat al-fitr, it is to be paid in the land where the person from whom it is due is present, whether his wealth is there or not. End quote.
As for paying zakaat al-fitr in cash, we have explained in the answer to question no. 22888 that it must be given in the form of food, and that paying it in cash is not acceptable.
So you must try to give it in the form of food. If the poor person refuses the food and asks for cash, then there is nothing wrong with your giving it as cash in that case, because of need or necessity.
And Allaah knows best.