inter-changeability of materials

Plese i need help.

I created three materials in a FFF class because of inter changeability. All the materials have the same unit of measure. When recording order unit in one of them, the system is displaying '' Error message because of the difference of order unit  in one material of FF class '' .This is not normal   because i do not need to have the same order unit on all the materials of FFF Class.

Thanks for your help.

Message was edited by: Caetano Almeida - Personal information removed.