What is the expiatory payment (fidya) due from one who is unable to fast on Mondays and Thursdays?

If a person intends to fast on Mondays and Thursdays voluntarily but does not fast them, no expiation (fidyah (definition)) is obligatory upon him. If he does not fast due to an impediment, the reward for his fasting is written for him as long as he intended it and was unable to fulfill it, based on the saying of the Prophet, peace and blessings be upon him: "If a servant falls ill or travels, the like of what he used to do when he was healthy and at home will be written for him." These two days are among the days for which fasting has been encouraged.