If he takes leave based on his saying that he is going for ‘Umrah, and in fact he does not intend to go, then this is lying. If that results in him getting leave to which he is not entitled, then the salary that he gets for this period is haraam wealth and is not permissible for him.
The one who does that has to repent to Allaah and go back to work.
With regard to the effect that has on his fasting and prayer, they are valid, but it indicates that he is not doing them as Allaah enjoined. If he was really praying as Allaah has enjoined, then his prayer would be preventing him from doing evil deeds, as Allaah says (interpretation of the meaning):
“Verily, As‑Salaah (the prayer) prevents from Al‑Fahsha’ (i.e. great sins of every kind, unlawful sexual intercourse) and Al‑Munkar (i.e. disbelief, polytheism, and every kind of evil wicked deed)”
Undoubtedly sins such as lying, deceiving, swearing, insulting others etc detract from the reward of the one who is fasting, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not give up false speech and acting upon it and ignorant conduct, Allaah has no need of his giving up his food and drink.” Narrated by al-Bukhaari, 6057.
It was narrated by al-Tabaraani in al-Mu’jam al-Sagheer and al-Mu’jam al-Awsat with the wording: “Whoever does not give up obscene talk and lying, Allaah has no need of his giving up his food and drink.” Classed as hasan by al-Albaani in Saheeh al-Targheeb wa’l-Tarheeb.
False speech has also been interpreted as referring to lying. This points to the abhorrence of lying when fasting, and shows that he is exposing his fast to rejection and lack of acceptance.
It says in al-‘Awn al-Ma’bood: “If he does not give up” means, if he does not refrain from. “False speech” means lying. “Allaah has no need” – Ibn Battaal said: This does not mean that he is enjoined to stop fasting, rather what it means is a warning against false speech and the other things mentioned. … Ibn al-Muneer said: Rather it is an indication that it will not be accepted. Ibn al-‘Arabi said: What this hadeeth means is that he will not be rewarded for his fasting … This hadeeth was quoted as evidence to show that that these actions detract from the reward of fasting. End quote.
See also the answer to question no. 50063.
And Allaah knows best.