> should know that TaskA requires 5 hours, and TaskB requires 1 hour.

I contest this. This assumes the manager is also some sort of software engineer plus I don't understand why would there be any negotiation: a project asks for an estimate and we give it based on how much we feel it's going to take, there is no negotiation; either the project manager accepts it or reduces scope and we make a new estimate, my manager is uninvolved.

I suspect this might be a work culture thing: in my country managers aren't timekeepers, their responsibility is making sure we have all the conditions we need to be productive and be "invisible" otherwise.