So I managed to build the Invoice, client, purchases and items tables and get them to work fine but I'm stumped with the credit notes. The Credit note table is related to the Invoices by Invoice number and thence to the Client table by the same relationship. If I relate to the line items table by invoice number I alter the invoice and stock. Do I have to build a credit line items table to lookup the values from items without affecting the stock, ( as the credits are for fresh produce any returned items are fed to the pigs and do not go back into stock).

Thanks for any help

Can you tell us how the Credits work in real life? Is it just a lump sum dollar amount that's applied to an Invoice, or are they broken down by product?

They are individual items from the invoice. If 20 boxes of apples are delivered pre-packed in 1.5kg units, with eight units per box, there may only be 6 packs returned out of the 160 delivered etc so we have have to be able to credit 6 units

I'd suggest making your credits be just Line Items that have a negative value and/or that have a Type "Credit" (vs. "Purchase"). This way the data entry could be done within the Invoice-Line Items structure.

If your Items table is supposed to keep an updated Qty on Hand based on purchases and credits, this can be done by counting the Line Items of each Type separately.