Refunding money to a customer

[Stroller <..hidden..>]

Hi there,

A while back, when working in a customer's restaurant on his laptop,  
I spilled a drink across the keyboard. My insurance company paid  
promptly, and sent a cheque directly to the customer who replaced the  
machine.

I have a £100 excess on my insurance, however, and would like to set  
this against invoices (long) owed by the customer.

I would assume to do this by creating a new invoice on the customer's  
account and enter a new item of value -£100 (say under my  
"miscellaneous discount" service) - is this the correct way? I'm not  
sure if it's legal to have an invoice of negative value? (in the UK)

Ideally I'd like this £100 to show up amongst the other payments &  
credits shown in a statement of account which I'd like to send to the  
customer (see my other post). However Jigme Datse's post of 13 March  
2008 16:06:55 GMT (SUBJECT: Re: statements, credit notes,  
overpayments, budgets, tax corrections) suggests that overpayments on  
one invoice are not automagically credited to other invoices on the  
same account, so perhaps my approach is wrong.

Stroller.