Capturing "Amount origin" for GST 0% and GST Exempt invoice line


My client in Singapore needs to caputer amount origin even from non-taxable AP invoice/ AR invoice transaction. So I setup the following tax group and code.

Sales tax code: GST 0%, GST 7%, GST Exempt, GST N/A, GST OS

Sales tax group: GST 0 %, GST 7%, GST Exempt, GST N/A, GST OS

Item sales tax group: DFLT (contains all sales codes)

By above setup, AX calculates correct sales amounts but it seems that there is no way to capture amount origin for non-taxable invoice line. Does anyway know how to capture origin amount for each sales tax code?

Thank you,