Record an external payment for a future invoice

Record payment from a customer that occurred outside the Ezypay system (for example an over-the-counter cash transaction) on a future invoice of a subscription.Ensures payments collected in advance externally from Ezypay can be reconciled and marked as paid. The future invoice will be issued immediately in advance as a paid invoice, with an external payment type recorded against it. Fee's will be allocated to the merchant, unless it was not attempted yet.