I have the same issue on this report and I'm looking for a workaround.

Having IN (positive) and (OUT) negative entry that sums up as absolute numbers can lead to misunderstanding.

As I understood that by grouping on this report, it just sums up all moved quantities indipendently if it is an IN (positive) or OUT (negative) move. It follows the logic of a  double-entry inventory (From->To), according to were the movements comes from and goes to, it is alwayss positive. 

An increase in the WH/Stock is also a decrease in Partner/Vendor for receipt  and a decrease in WH/Stock is an increase in Partner/Customer in case of sales. So each movement IS both positive and negative depending on the direction of the move against he WH involved(from->to or viceversa).

To check the stock (avaiable quantity) you have to run the Stock report.

BUT what if I would like to know the balance of product movements in a specific wharehouse (WH/Stock for example) in a given date and gouping it by product?

and most important for me, how to get the report/list of  LOTs stock avaiability (on hand) amount with all moves that happened to create that stock grouped by product and  lot/serial numer ?

I tried to run the moves report and grouping it by category, product and lot/serial number, the list of movements is perfect, except for the sum, because considering every quantity as positive, it gives the wrong total number (it should decrease the red numers)

Is there a workaround to have a sum of product movements that considers the green numbers as positives and the red numbers as negatives in the moves report?

This could make it easy to do a lot of check reports, by a range of date, goups of lots, and other possible movements grouping and filtering.

Thanks

Edit: I solved the issue with the stocks by lot using the Quants menu (you need to add it manually in your menus). I fileterd the report by WH/Stock, and grouped it by Product and it gave me the stock by each Lot and the button to check the cronology of it (moves that sum up to the stock, on another screen)