GST on sponsorship from foreign body corporate

GST is applicable under the Reverse Charge Mechanism (RCM) as per Section 9(3) of the CGST Act.

The receipient trust must pay IGST on the sponsorship amount because the service is an import of services (foreign suppplier = Indian recipient).

Your understanding that RCM won't apply because the sponsor is unregistered is incorrect. RCM applies even if the foreign supplier is not GST - registered.

2. Admission fee/membership fee from a body corporate situated in SEZ in a different state:  The transaction is treated as an inter-state supply, so IGST is applicable.

If the SEZ unit is deemed an "export" (zero-rated supply), the trust can claim exemption/refund under LUT, provided the spply is eligible for SEZ benefits.

RCM may apply if the service falls under notified reverse-charge ategories; otherwise, the normal forward charge (FCM) would be used by the SEZ unit if it were GST-registered.