Configuration for Free of Charge service invoice

Hi,

As per our present process, we are creating the service sales order in which we add the material which we have used for service and also the repair charges , and this document type is not relevat for delivery for this we are using item category TAD, so directly billign is created after SO, this process is working fine, the accounting entry at billing is as below

Customer account debit for gross amount

Material / Labor chrages account credit for material amount

Tax account are credited as per tax value,

Now as per this we want to configure the free of charge service only insted of customer accoun debit we want to give some other G/L account the gross value to be hit to that account, what configuration is required for this, can we do thsi by keeping the sales order type and pricing same, and only creatinng new billing type. or what to be done to get this.

regards,

zafar