Change unit cost at Inventory transfer_ Project Accounting

Good Afternoon:

I have a question of project accounting module. I ´ve created a time-material project and need make some inventory transfers, the user manual and every instruccions says " select the item number, unit of measure, quantity and unit cost". BUT When I try to change the cost that appears by default , (it can be for downward adjustment or increasing adjustement)  I am receiving the following error message:

"the unit cost  can´t change for downward adjustment"

I have checked all the permissions on configuration of project accounting module, but I don´t get any change.

Your help with this would be greatly appreciated.

Thanks

Fabiola Moreno.