Hi
If your pricing condition is manual in invoice and order then it has no link with PGI. In standard settings user can change price before you post the invoice. For linking this with PGI you have to go for userexit coding. Ask your ABAPer to write code in MV45AFZZ for sale order and RV60AFZZ for invoice. There are pricing prepare userexits in these two programs. Your ABAPer must know how to do this and if not then he can Google it.
Thank$