This was at the beginning of Islam that people had the right to pay money instead of fasting but then this was abrogated and only those who are too old or too sick to fast and can never make up, they are the ones addressed to feed a poor person.

What is the meaning of these words from: Surat Al-Baqarah 2, 184; “And upon those who are able [to fast, but with hardship] a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess – it is better for him. But to fast is best for you, if you only knew.