Selling weight-reduced Material

Hi Experts,

I need your help to get through with this problem, I would appreciate if you can give me a business model along with solution.

selling material A1 from X country to Y country.since its a liquified material it was weighing 100 lbs before the shipment starts in X country.By the time it reaches Y country this material's weight has been reduced(evaporated) and now its 80 lbs.How to bill this material in billing document and how could we calculate the freight for this type of materials?

Thanks in advance,

Kanna.