The amount of food that is given as fidyah (ransom) for a missed fast due to old age is estimated as equivalent to half a saa` (1 saa` = 2.03 liters). In Canada for example, it is estimated as seven dollars in cash.
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states the following: The amount of food (volume or weight) to be given is traditionally estimated as equivalent to half a saa` (1 saa` = 2.03 liters), but for our consideration it is sufficient to estimate it as the equivalent of one full meal. In the Canadian situation it can be estimated as seven dollars in cash. But this varies according to time and place.
If the person is in old age and he is looked after by his son, then if his son pays it on his behalf that would be sufficient; if he refuses to pay, then the person is excused if he has nothing to pay with.