Allah says in Quran:
فَمَنْ شَھِدَ مِنْکُمُ الشَّھْرَ فَلْیَصُمْہُ
Meaning: ‘’Therefore, he who witnesses this month (Ramadan) must fast in it.’’ [2:185] Al-Baqarah
The month of Ramadan is reserved for fasting. If a person misses a single fast of Ramadan due to his misfortune, then a major sin has been written in his book of deeds. Even if he fasts for a lifetime in return for this one fast, he will not gain the blessing of fast of Ramadan.