What is the ruling on postponing making up for missed fasts until after the following Ramadan, and is it permissible to postpone expiation (kaffara) as well?

It is not permissible to delay making up for missed Ramadan fasts until the next Ramadan without a valid excuse. Whoever intentionally does so is sinful and is obligated to offer an expiation for the delay along with making up the fasts. They must make up the fasts before the advent of the next Ramadan. The fidyah (expiatory feeding) does not multiply with repeated delays.