Is it permissible to spend zakat funds on the needy poor in exchange for their performing public service works such as planting trees and cleaning streets, knowing that the giver of zakat does not personally benefit and that the money goes to the needy without purchasing materials?

It is not permissible to withhold zakat from its rightful recipients, the poor and the needy, and pay it to them as wages for work. Rather, it must be given to them based on the descriptions upon which the Lawgiver conditioned their entitlement. Thus, the poor person is entitled to it due to their poverty, and the needy person due to their neediness, and not in exchange for assigning them a task.