Farming Levy


I have a client who is a farmer on Cash accounting. 

I have posted a sales invoice for crop sales that has zero rated VAT. Now the issue is the levy, when I put a minus on the same invoice with 20% VAT and match the bank money received. Sage accounting doesn't seem to like this and reverses everything on the VAT return.

I have then tried a sales credit note against the sales invoice and I get the same outcome. 

How can I fix this? As I need to reclaim the VAT on the levy.