We have been using the supplier invoicing and deliveries interface for some time and I should have posted this earlier...

The supplier credit note feature that is used to generate a credit not for returns - doesnt really work.

Firstly it use the products retail price which is incorrect - suppliers only credit you for the price you paid - ie NETT price 

Its almost impossible to make the credit note match the true value because qty affects the line item price conside an item purchased for $98.86 ex GST - 

You sell it at 2.00 a tablet (there are 105 tablets in a pack) creating a credit note for the entire pack will create a value of around 210 dollars... the true line item value is more like 0.9418095238095238‬ however the line item is rounded to 0.94 - creating a total value of 98.70 - My advice would be to allow an optional tax exclusive item value total so we dont have to use a qty x method to calculate the credit value - or drag the nett item value straight from the supplier product price if it exists.