Need solution

Question

We issue Purchase Order in DRY TONS ( Unit Of Measure)
We recieve material in WET TONS ( Unit Of Measure) wich contains Dry Material + Moisture.
But the payments are done in DRY TONS.
Again we do Goods Issue in WET TONs
Problem here is they get to know the moisture content after 1 week of Goods Receipt due to lab procedures they follow.
My Approach:
first i create PO for 1000 DRY TONs.
when i get the first delivery of 100 WET TONs, But I do GR for 100 DRY TONs.
But after 1 week, we get the moisture content as 10%. so then can we go back and change the GR quantity from 100 DRYTONs to 90 DRYTONs.
Problem is
If the invoice for that month is already posted at month end, and the results of moisture come after that month, then how to handle the situation.
Is my approach correct or is there a better efficient solution.

Answer 1

Thanks Charlie for the reply,
Actually currently in their legacy system , they are handling it in two ways based on the type of transport.
Case 1:
If it i though rail usually, it takes longer time for the material to come in, so , by the time material comes in we already have the test results ( which is done at the vendor location itself, by a third party)  and actual moisture content. So we pay the correct amount irrespective of what date it arrives.
Case 2:
If it is coming through barge, the barge comes to our plant almost the same day, usually the result of the lab test ( which is done at the vendor location itself, by a third party) arrives 5 or 6 days afetr we unload the material.
In this case the actually payoff the vendor ( if the lab results didnot arrive by the payment date ) with a standard moisture content ie. 10%. but later if it was found that, the moisture is 12 or 13 or 14 % instead of 10. then based on previously agreed contract, they  add or substract the money in the next month payments.
Please let me know if i am not clear.