Billing value not to be impacted of first 2 line items while adding the third line item in sales order for milestones

Hi Experts,

When we are creating the sales orders there are 2 materials which we are entering land and construction material  but when we are entering the line item 30 it is impacting the billing value in billing tab in sales order line item level for both the items and it is changing the billing value and increasing the billing value for both the items we dont want to change the value in billing tab in sales order for both the line items . can you suggest a solution for the same . Thanks for the help