You can’t do that when the clients are internal ones.

The title says customers and his comment mentions: "They want it cheaply so there are no resources to test."

I have never heard of internal customers, I have heared of internal projects ;) . But then the cost are not discussed, deadlines are.

And it's still the mission of the product manager to coordinate features to build.

In case it's only 1 developer, he has to take the lead and say pro's and cons.

An internal project budget is to prevent other teams from just endlessly requesting features from the internal developers, i.e. they cannot treat the internal development team as free. It also prevents them from hoarding the developers.

Most of the time the billing is just budget shuffling.

But it needs input from the product manager.

Budget shuffling or not. ( I didn't say work is perfect, but experienced people can make it more doable)