What you have done – signing for the days when you are absent from work – is undoubtedly a haraam action, because it involves lying and consuming peoples wealth unlawfully.
Allaah says (interpretation of the meaning):
“O you who believe! Eat not up your property among yourselves unjustly” [al-Nisa 4:29]
And the Prophet (peace and blessings of Allaah be upon him) said: “Truthfulness leads to righteousness and righteousness leads to Paradise. A man will keep speaking the truth and striving to speak the truth until he will be recorded with Allaah as a siddeeq (speaker of the truth). Lying leads to immorality and immorality leads to Hellfire. A man will keep telling lies and striving to tell lies until he is recorded with Allaah as a liar.” Narrated by al-Bukhaari, 5629; Muslim, 4719.
The salary that you have taken in return for these days should be given back to your workplace; it is not permissible for you to keep it because it is haraam wealth.
The Prophet (peace and blessings of Allaah be upon him) said: “Every body that is nourished with haraam food, the Fire is more fitting for it.” Narrated by al-Tabaraani from Abu Bakr; classed as saheeh by al-Albaani in Saheeh al-Jaami, 4519.
Al-Tirmidhi (614) narrated from the hadeeth of Kab ibn ‘Ajzah: “There is no flesh that grows from haraam food but the Fire is most suited for it.” Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
You have no right to give this money in charity, because wealth can only be given in charity when its owner is unknown or it is too difficult to reach him.
This action is not permissible according to pure Islamic shareeah, because he is acquiring money by means of lying and deception. Acquiring money in that manner is haraam and must be denounced and warned against.
May Allaah grant us sufficient provision and keep us safe from that.