When creating an invoice or purchase and you realise that an item is missing from your sales/purchase codes, you currently have the option to create a new sales / purchase code at that point.
However, you have to guess what codes you have already used and often get told (this sales code already exists).
We need a drop-down list when entering the new sales / purchase code itself, so that you can see which ones are used (for example if you use a letter and sequential number).
