If you dont fast (even when you are fit enough to) and instead you pay the poor for not keeping the fast is that a sin
Dont fast and pay kaffara
In the name of Allah, We praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright. We bear witness that there is no one (no idol, no person, no grave, no prophet, no imam, no dai, nobody!) worthy of worship but Allah Alone, and we bear witness that Muhammad (saws) is His slave-servant and the seal of His Messengers.
Allah says in the Holy Quran Chapter 2 Surah Baqarah verse 185:Ramadan is the month in which the Quran was sent down: this Book is a perfect guidance for mankind and consists of CLEAR teachings which show the right way and are a Criterion of the Truth and falsehood. Therefore, from now on, whosoever witnesses it, it is obligatory on him to fast the whole month. But if one be ill or on a journey, he should make up for the same number by fasting on other days (of the year).
Fasting is one of the obligatory acts and pillars of Islam, and thus it is obligatory on every muslim to fast the whole month of Ramadan. If someone is eligible to fast and does not fast, then he would have broken one of the obligatory commands of Islam, and it will be considered a manifest sin.
The kaffarah or atonement of feeding one needy person for each fast missed, is a concession only for those people who are perpetually ill, or extremely old and weak, and are thus unable to keep the fast at all in Ramadan or any other days of the year. But if one is young and healthy, and does not fast for no valid reason, it will be considered a grave sin in the sight of Allah Subhanah.
Whatever written of Truth and benefit is only due to Allahs Assistance and Guidance, and whatever of error is of me. Allah Alone Knows Best and He is the Only Source of Strength.
Your brother and well wisher in Islam,