unpaid invoice.

Hi all,

I am new to this forum also to the business, and have a question about unpaid invoice. As I did some mistake of giving customer an invoice (instead of pro-forma) cos hoping that they will pay me the money in advance before the goods will be delivered (as our terms of business is full payment have to be received before delivery)but obviously they refused to pay for their order and claimed that they need to see the goods first before paying...so I have to cancel that order accordingly.

My question is, will it cause me any problem in case of self assesment/ VAT return etc.? I just thinking.. what if that customer make use of invoice I issued to claim their VAT back, do you think can they do that? what should I do then?

Any responses are very welcomed, and thanks very much in advance.

Kik