Hassan
You won't find a SAP solution that allows you to have a different receipt quantity for the materials and freight. You could use inbound shipment costing and build a pricing more complex pricing procedure to calculate the freight price but the received quantity will always be the same.
I believe you should receive the quantity loaded by the vendor and handle any changes in moisture in inventory after the goods receipt. I'm sure your vendor expects to get paid for what they loaded and that is where the regulation you mention comes from.
Why would your freight vendor expect payment for anything other than the weight that was loaded at the vendor? Do you pay them more if it rains during transit?
Part of being an SAP consultant is preventing scenario's from becoming needlessly over complicated and this is one of those times.
Regards
Robyn