Follow this format (replace all highlighted fields with what you have in your schema):
a. Repeating group: xdXDocument:GetDOM("MySharePointList")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW/d:CommissionRate[../d:SalesPerson = current()]
b. Non-repeating group: xdXDocument:GetDOM("MySharePointList")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW/d:CommissionRate[../d:SalesPerson = xdXDocument:get-DOM()/my:myFields/my:SalesPerson]
If you still have issues, please attach a copy of your XSN and I'll take a quick look.