All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
In principle, it is impermissible for the employee to take money for the work days on which he was absent unless the company regulations or his contract permit it as is the case when the employee takes a paid absence of leave for a specific number of days.
What we understood from the question is that your company regarded those days when you were absent as a paid leave (a sick leave), considering that you were sick and your contract entitles you to take a paid sick leave. However, you were not actually sick. Therefore, you are obliged to repent of such a lie and it is incumbent on you to return the money of this sick leave to the company because you are not entitled to it. You cannot simply offer this money in charity; you have to return it to the company.
The fact that you wished to devote your time in Ramadan to worship does not justify telling a lie like that. You could have taken a leave instead of lying. In fact, what you have done is considered cheating and falls under the heading of false speech. Allaah, The Exalted, says (what means): {. . . So avoid the uncleanliness of idols and avoid false statement.} [Quran 22:30]
The Prophet, sallallaahu ‘alayhi wa sallam, said: “He who cheats us is not one of us.” [Muslim]
He, sallallaahu ‘alayhi wa sallam, also said: “Shall I inform you of the gravest of the major sins?' He repeated it three times. We said, ‘Of course! O Messenger of Allaah!' He said: ‘Associating with Allaah, undutifulness towards one’s parents,' and he was reclining so he sat up straight and said, 'Beware of false speech and false testimony.' He repeated it so many times that we wished that he would stop.” [Al-Bukhari and Muslim]
Ar-Raaghib commented on the Hadeeth saying, “False speech (the Arabic word az-Zoor in the Hadeeth) refers to lying.” Al-Haafith Ibn Hajar wrote, “Az-Zoor is to describe something contrary to what it really is.
If speech is described as 'Zoor', it includes lying and falsities in general. It is also used to refer to false testimony exclusively. It may also be used to describe someone's actions as 'wearing two garments of Zoor,' to denote deception, and in reference to artificial hair as false hair.” [Fat-h Al-Baari]
Allaah Knows best.