Preventing Checkout with a minus total?

[RonnieR]

Hi,

 

I've set up an easy (for me!) system from a contribution to credit individual customers accounts with an amount that I can choose.

 

In testing it I find that I can checkout with a minus figure, i.e. the store site owes the customer.

 

I have searched throught he documentation and forums but can't find a way to prevent this through Admin or a code snippet.

 

Ideally it would be good if the minus figure was left in the customer account as a credit for future use.

 

I'd be grateful for any advice.

 

Regards

 

RonnieR