There is no requirement in GST law that every movement of goods have to be accompanied by Tax Invoice. In other words, only where outward supply of goods takes place, tax invoice needs to be issued.
When goods are sent for demo/trial, it is not an outward supply of goods. In such scenario, the GST law provides for Delivery Challan to be raised in Rule 55. The same is reproduced as below:
55. Transportation of goods without issue of invoice.-
(1) For the purposes of-
(a) supply of liquid gas where the quantity at the time of removal from the place of business of the supplier is not known,
(b) transportation of goods for job work,
(c) transportation of goods for reasons other than by way of supply, or
(d) such other supplies as may be notified by the Board,
the consigner may issue a delivery challan, serially numbered not exceeding sixteen characters, in one or multiple series, in lieu of invoice at the time of removal of goods for transportation, containing the following details, namely:-
xxx
This provision maybe highlighted in the appeal, along with factual position and also documentation regarding the demo/trial i.e. any e-mail communication, confirmation from other party etc. to prove that it was infact only for demo/trial purpose and not for outward supply.