Error in Delivery Note UOM Conversion

Let us assume a UOM conversion 4 Nos = 1 Square foot for an item x.

I had created a delivery note for a item x of quantity 1000 square foot.

Now i want to return 500 Square foot in Nos (which is 2000 NOS). And i make a delivery return of -2000 Nos.

Now i again want to return 100 square foot in Nos (which is 400 Nos). And when i creating a delivery return against a first delivery note it shows 1000 square foot(2000 nos - 1000 sqft). But it is worng, it has to show -500 sqft.
It dosen’t consider about UOM Conversion.

Is someone know how to solve this problem…