Hi Prashant,
When you are posting a FICA document, whether it will be posted as gross amount or net amount, is decided from SPRO Configuration-'Maintain Central Settings for Posting'
If the Setting, 'Receivable Amounts are net amounts' is not ticked, then the document posted is gross which comprises of the tax component.
In your scenario, as the disconnection charge is a configured charge and is constant , so can you not configuring the charge with the tax component, say 113 CAD instead to 100CAD in SPRO Config.
Alternatively, you can enhance event 1110, to add tax items.
Hope it helps.
Thanks,
Amlan