Is taking an additional amount from the donor in exchange for dedicating time to the process of delivering water and digging wells considered permissible or forbidden?

It is not permissible for someone who collects money from donors to take anything from it for himself except with their knowledge, because he is their agent, and an agent does not act except with what he has been authorized to do. Ibn Qudamah is of the opinion that an agent does not have the authority to act except as required by the principal's permission. It is stated in "Fatawa of the Permanent Committee for Issuing Fatwas" that it is not permissible for those in charge of collecting charitable donations to give employees from these funds because they are agents for the donors, and they must deliver them to the rightful recipients. Moreover, this action involves lying if the donor is informed of a higher cost. Therefore, it is obligatory for you either to do the work seeking reward from Allah, or to inform the donor of the truth so that he can make his decision, or to stop this work.