Hi @Adele552266,
The first step is determining who is making the supply and who needs to issue the invoice.
If the agent is invoicing the sponsor in their own business name on behalf of the athlete, this would typically be considered an agency arrangement. However, the GST treatment depends on whether the agent is acting as a disclosed agent or an undisclosed agent, and whether they're making the supply themselves or facilitating a supply by the athlete.
When an athlete isn't registered for GST, they can't charge GST on their services. If the agent issues an invoice to the sponsor on behalf of the athlete, that invoice shouldn't include GST if the athlete is the one actually making the supply to the sponsor. The agent can't include GST on supplies made by someone who isn't GST registered, even if the agent themselves is registered for GST.
After the agent receives payment from the sponsor, the typical arrangement would be for the agent to account to the athlete for the funds received, less their commission. The agent would then invoice the athlete separately for their commission services. If the agent is registered for GST, they would include GST on their commission invoice to the athlete, as this is the agent's supply of services to the athlete.
The alternative structure would be for the agent to act as principal (making the supply themselves to the sponsor), then acquiring services from the athlete. This changes the GST obligations significantly, particularly if the agent is GST registered.