Praise be to Allah.
The Shaytaan keeps on making falsehood appear attractive and gradually makes a person fall into the worst of evil deeds and the gravest of major sins, namely shirk (associating others with Allaah), thinking that he is doing himself a favour. How can a person try to avoid fasting for two consecutive months by disbelieving in Allaah the Almighty, when Allaah has forbidden Paradise to the one who meets Him as a kaafir? The one who does that is like one who jumps out of the frying pan into the fire. He flees from one thing and falls into something that is far worse.
The trick did not benefit him by relieving him of what was obligatory upon him, because it is a haraam trick, indeed it is the worst of all haraam things. The basic principle according to the scholars is that a trick does not waive an obligation or make something haraam permissible. Can a person be sure that if he commits this major sin, Allaah will give him time to repent and come back? Is it not possible that the last moments of his life may be when he has announced his kufr, Allaah forbid? Then he will be one of those whose deeds are lost in this world and in the Hereafter, and such are the dwellers of the Fire in which they will abide forever.
Does he not fear that Allaah will punish him for this evil action by preventing him from repenting and coming back to Islam?
“So when they turned away (from the path of Allaah), Allaah turned their hearts away (from the Right Path). And Allaah guides not the people who are Faasiqoon (rebellious, disobedient to Allaah)”
To sum up: The one who has done this has done something serious that makes the flesh of the believers crawl. Praise be to Allaah Who has enabled you to repent. We hope that Allaah will accept your repentance and forgive you your sin.
Part of perfecting your repentance is to do a great deal of righteous deeds, remembering Allaah (dhikr), reading Qur’aan, asking for forgiveness, acquiring knowledge and teaching it, giving charity, calling people to Allaah, etc. There are many kinds of good deeds, so strive hard and may Allaah forgive you. Allaah says (interpretation of the meaning):
“And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)”
The punishment for the one who apostatizes from Islam is death, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever changes his religion, execute him.” Narrated by al-Bukhaari (2017).
The majority of scholars (including the Hanafis, Shaafa’is and Maalikis) are of the view that this punishment is waived in the case of one who repents and comes back to Islam, which is what applies in your case.
See al-Mughni (9/18) and Sharh Muslim by al-Nawawi (12/208).
With regard to the ruling on masturbating during the day in Ramadaan, it invalidates the fast and what you have to do is make up that day only, not offer the expiation for having intercourse. In the answer to question no. 50632 we have explained that the expiation for breaking the fast is only required in the case of intercourse.