Average cost update is not averaging out to the entire quantity

Summary:

Hello Experts,

We have requirement like Item A - receipt qty - 100 with transaction cost of 10, value is 10*100 = 1000

In review item cost - unit cost shown as 10,

After some days, they have received custom charges of 100 qty is 500, this charges should be added to item cost as 5 for each therefore 100*5 = 500, final unit cost should be 1500/100 = 15

For the custom charges when we perform Average cost update of Amount Change as 500, it directly applies to the Item unit cost as 500+10 = 510, inventory accounted as 510*100 = 51000 this is not as expected.

Can anyone help, how to perform average cost adjustment which distributes the cost evenly to all the onhand quantity