What is the ruling on taking a percentage of donations, ranging from 20% to 35%, as administrative and operational expenses without the donor's knowledge?

It is not permissible for the office to take more money from donors than what is stipulated in the contract. Otherwise, it must clarify this to the donors. If they agree, then there is no harm; otherwise, it should apologize for receiving the donations. The intention to benefit the poor does not justify deceiving the donors. The employee's work on the project is lawful (halal), and his salary is lawful. The fact that the office's funds are mixed (containing both lawful and unlawful elements) makes transactions permissible as long as the unlawful does not predominate.