Automatically update seller commission field in product node

1) when commission rate has been changed
2) when product is uploaded with Node import module

I noticed the seller commission field does not update when I changed the commission rate. The field still contained the old rate. And also the commission field of products uploaded with Node import module does not calculate. The seller commission field remains $0.00 even after I clicked on edit and resave the node.

My question is, how does one recalculate the commission field for a product node since it doesn't update even after "edit and save"? Thanks.