Saleslines on order does not update

Hey,
I'm using events to modify some of the line fields on salesorder. The event runs using codeunit. O have subscribet to onaftermodify event, and when the fields get updated the values on the page doesnt change. When i use SalesLine.modify, the values on the pages gets updated but whem i post the order i get an error saying "Unable to change an earlier version of the Customer record. The record should be read from the database again. This is a programming error". Does anybody have a workaround or solution? Thanks