I do not think system can maintain free item automatically.. is there is any fixed business rule or variable ?.. even if I suppose you have fixed business rules for material that vendor give as free always. then you can put validation in PO creation .using ME_PROCESS_PO_CUST with item processing method.. I think you can make system to fill item level data too using this BADI..
Another hint you have to maintain these rule in some custom table or think of using existing fields in material master, info record ..etc