Calculate tax for 0 amount Sales Order

We have a concept of FOC items free of cost items, but as per UAE vAT law we need to calculate VAT(5%) even if we provide goods as FOC at the fair value selling price.

Now since the Sales Order total for FOC is Zero value ( this is applied via manual modifier by updating the selling price to 0) the VAT calculated is also 0.

We want to automatically calculate the tax and don't want any manual intervention for VAT amount to be added manually.

How can we handle this case.. ?