It took me some time, but now I am sure that there is a bug in the excelexport.
While Uploading the attribute with ID "MatchingVendorStockNumberID" is transmitted.
While exporting the same field is filled with "MatchingVendorStockNumber" is transmitted.
This must cause an error! so it is impossible to export, modify and then reimport the file without destroying the matching pairs.
PLEASE RAPNET FIX THAT!! it is only two letters!!!