Now, the Project Manager has assigned a cost to this ressource. But the economy of the project is false because the old real work allocation was based on a cost=0€.
I just did the test and it worked well.
I repeted it on demo, and still works well :
Create project "no cost".
Create resource "no cost", with no cost, defaut function "manager".
Allocate resource "no cost" to project "no cost".
Add activity "no cost activity" on rpoject "no cost".
Assign resource "no cost" to "no cost activity" with function manager.
Add one real work day for "no cost" on "no cost activity" in the past (20/02) now (27/02) and in the future (06/03).
Real cost of activity is zero.
Add cost for resource "no cost", function "manager", 100€/day.
Cost for activity is automatically updated to 300€, and so is the project.