GST sale at concessional rate of 5% to institution funded for research and development

We are traders of laptops and computers in Karnataka. we purchase and sell at 18% gst. We have raised a sale invoice to an institution which is funded for research and development purpose and which is liable to concessional GST at 5%. I would like to know how to account for the difference in gst (purchase @ 18% and sale @ 5%) in our books.