Multiple Customer Advance Invoices

Hi Team,

I have a Customer Order having 2 Order Lines with below amounts:

1.  Part A - 10000 (Net Amt), 11800 (Gross Amt)
2.  Part B-  5000 (Net Amt), 5900 (Gross Amt)

Now I want to create 2 Advance invoices for it.

In Create Advance Payment Invoice window, 

For first Advance Invoice, I have entered Invoice Net Amt as 10000 & Apply Tax check box is selected so tax is getting calculated as 1800. Gross Amount is 11800 (Getting Correct Value)

Now the issue is when I am creating second Advance Invoice, entered the Invoice Net Amt as 5000, it is calculating tax amount as 1800 taking into consideration Tax Base Amount as 10000 whereas I require the Tax Base Amount as 5000 . It is not allowing to change the Tax Base Amount which is auto fetched.

Correct Gross amount for second advance invoice should be 5900 but I am getting 6800.

Request you to please help me with this.

Thanks in advance

Rakesh