In projects module, how can an invoice only can be raised with Tax portion itself but not the base a

Can anyone describe the process and navigation steps for creating an project invoice in projects module  with Tax portion itself but not the base amount. Here we had a business requirement in such a way that we had some free shipping items to be booked against the customer only with tax portion from project module generated invoice. This invoice should flow from project module only that too for tax portion only but not considering the base amount.

if I book the tax portion in base amount portion it will hit in my Percentage of completion as base amount only but not the tax. so can anyone pls. suggest a solution for this type of transactions booking in projects module.