Issues on Materials valuation, initially recorded based invoice but valued in system on average approach?

How do you approach some issues on inventory valuation, such that, there will be spotted issues on the purchased cost of inventory versus its valuation (that purchased materials) on the system's weighted average valuation approach. So that materials are valued initially based on invoice price, and recorded in system which were re-valued based on average approach, therefore, it came with an issue of valuation differences between the initial purchased invoiced cost and the system's average costing especially when that is brough materials inventory to In Process Costing. What is your basis of valuation of Inventory in the Process Costing? based on Invoice Cost when purchased? or based on the average cost as computed in the system average approach?