If, for example, a bill has billed both fees and disbursements and a partial payment paid off full amount on Disbursement + tax, and partial payment on fee as in below:

Once you write off, it will automatically prorate the GST/PST based on the ratio of partial payment and remaining balance. The write off in client ledger will show as write off on Fees:

The credit of GST/PST automatically allocate to GL of GST/PST billed/collected as in below:

And this GST/PST write off will show on your GST/PST report as in below: