Return tax Calculation

Hi

I have requirement to differentiate the tax for Sales and return process for FP Billing type

We are having SAP IS Retail process and Billing document creates via aggregate sales, now i.e. Article X is sold with $10 with GST tax $2 and the same article return with -ve sign and return price is same as $10 and return GST tax $2,

My requirement is how to post the GST tax with two different G/L account one for sales and other for return in same FP billing type.

Currently it is appearing as GST tax as zero, as one is +10 and other is -10 and entire GST is posting to one GL account.

Please advise.