The only way you can pay off an invoice with a negative value is via the quick pay function. The allocate method will not work in such cases where you part pay.
You have a few choices now. Either you can pay the remaining £5 via the quick pay function (The initial £10 will allocate). Or, you can remove the invoice altogether and enter the full amount via Money > Manage Money > Money Out and then use the refund tab and this will mark it as a refund automatically. Finally, if you still wish to use allocations then you will have to create separate refunds to be able to allocate more than one payment.
I hope that this gives you a number of options.