Is it permissible to give money in lieu of fasting the missed days from the previous Ramadan, knowing that the postpartum period (nifas) will end a few days before Ramadan? Or is it obligatory to make up for them by fasting? And what is the amount of the fidyah if it is permissible?

As long as you are able to make up the fast, feeding the poor or giving out money is not sufficient for you. Rather, you must make up the days you missed, due to Allah's saying: "But whoever is ill or on a journey, then a number of other days." If you are able to make up the fast before the start of Ramadan, you must hasten to do so. Delaying the make-up fast until after Ramadan without a valid excuse is not permissible. If you delay the make-up fast without a valid excuse, then expiation (fidya) becomes obligatory upon you, which is feeding one needy person for each day. However, if your excuse continues until Ramadan begins and you are unable to make up the fast, then you should make them up after the following Ramadan, and there is no sin upon you nor any expiation.