What should I do if I cannot pay zakat al-Fitr due to not having enough money, and is it permissible to pay it after Ramadan?

Zakat al-Fitr is obligatory for a Muslim, for himself and for those whose maintenance is his responsibility, provided that it is in excess of his and his family's sustenance for the day and night of Eid. If he is able to pay it and does not give it out on time, it remains a debt upon him and is not waived, and he must pay it. However, if he was in hardship at the time it became obligatory, then it is waived for him, and if he later becomes affluent and pays it, he has done well.