When funding using invoice factoring, factoring companies rely largely on the verification process for establishing that the invoice is real. If it is a reoccurring billing situation this entails contacting the accounts payable department of your customer to verify; the amount of the invoice, the work has been completed and accepted, and that the remit to address properly shows the factor’s payment lockbox address. Normally this is done over the phone, or a fax, or by email.
Sometimes there are situations where a more formal sign off is required by the customer. If you only have one customer, or the billing model is somehow abnormal, then the factor will need the customer to sign a document stating that this invoice is proper and will be paid in full without any credits deducted, nor contesting of the payment.
So be prepared to the possibility of having your customer verify in writing that the proceeds of your invoice have been pledged to a third party (the factoring company). This is a normal course of business and is frequently used to fund accounts receivable.