Praise be to Allah.
The employee must do his job and his tasks in accordance with what was agreed upon in his work contract. If he falls short in that, he does not deserve his salary except what is commensurate with what he has completed of his work.
If the tasks are too many and there is not usually enough time to do them, then the employee has the choice between accepting that and continuing in the job, or not renewing his contract if it is of limited duration, or stopping working at the end of the month if the contract is on a monthly basis.
If he accepted what is stipulated for him, then he has no right to fall short in doing what is required of him.
What you have mentioned of showing that the work has been completed when it has not actually been done is a kind of deceit and consuming people’s wealth unlawfully. Allah, may He be exalted, says (interpretation of the meaning): “O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent” [an-Nisa’ 4:29].
The Prophet (blessings and peace of Allah be upon him) said: “It is not permissible to take a person’s wealth unless he gives it willingly.” Narrated by Ahmad (20172); classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (1459).
And he (blessings and peace of Allah be upon him) said: “Whoever deceives is not of me.” Narrated by Muslim (102).
So try hard to do the work as required, and strive to speak to those in charge so that they may reduce the number of tasks required. We ask Allah to help you and grant you provision by His bounty.
And Allah knows best.