No one can fast on behalf of another person. If a person is not capable of fasting in the month of Ramadan due to some reason, such as old age and or such an illness from which he has no hope of recovery, then he may offer fidya (monetary compensation) for each day he is unable to fast. (Imd?d al-Fatawa 2/151) (Radd al-Muht?r 2/119) These fidya payments are meant to feed a misk?n (person in need) for each of the fasting days missed and are equivalent to the price of feeding one person twice. If he wishes he can give fidya from the beginning of Ramadan for the entire month or he can give it at the end of the month. To give fidya before the month commences will not be correct. (Ahsan al-Fatawa 4/430,445)