All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The contract serves as the law of the contracting parties. Thus, the conditions stipulated by one of the contracting parties must be fulfilled unless they included unlawful terms. This is because the Prophet (PBUH) said (What means):" Muslims are on (i.e. stick to) their conditions except the conciliation which makes lawful unlawful and unlawful lawful." {Narrated by Al-Hakem.}
Therefore, if the employer and the employee agreed on certain terms, then none of them is permitted to violate this agreement nor to demand an allowance for the difference of working hours. And Allah knows best.