Currently, Purchase Receipt is required everytime you enter Landed Cost. However, there are landed cost that are being paid prior to receiving of goods. In this case, there should be an option to enter and release landed cost document without specifying Purchase Receipt number since the goods were not yet received. Upon arrival of goods and entered the corresponding Purchase Receipt, user must be able to link this purchase receipt with the landed cost previously entered.
Payment of landed cost prior to receiving of goods is very common in real world application.