Gst on custom milling

In Chhattisgarh state the Goverment is procuring Paddy at support price from farmers through various Co-opertive Societies. The Paddy so procured is then issued to Rice Mills for custom milling of paddy and Rice is delivered to Government through State Civil Supplies Corporation for sale through Fair Price Public Distribution System.

For Custom Milling the the service charges as fixed by the Central Government is paid on which GST is also paid by the Government.

When a rice miller mills additional quantity he is paid Protsahan Rashi  (Additional Encouragement Amount) based on quantity milled but the Government is not paying GST on this.

Please guide as to whether this Protsahan Rashi is not liable to GST ? If so kindly share relevant provision / notification. Thanks