Create New Module Called balance

Hello, I have a new top urgent thing now regarding vtiger CRM In our Accounting module we need to add several things to it, we need to link it to the Clients and the Contacts, and Invoice, and we need to add a field called Original Balance and another field called Current Balance. now when any invoice comes out for this particular customer the original balance will be subtracted from the amount in the invoice, and the new balance will be in the Current Balance automatically, and the invoice will take its details from the Module ( Services )