multiple payments including a refund

I have a lump sum receipt from a bank import to allocate to multiple customer accounts. This lump sum will cover invoices created for over 20 customers but also includes negative amounts as the customer is reclaiming money from certain accounts. How can I allocate the negative amount? (and in doing so increase the amount available to allocate). I also sometimes need to allocate an amount to a customer account where the invoice has not yet been created - Allocating to customer and trade debtors does not work in this instance. Any suggestions?

Thanks.