Requirement to bill customer before vendor invoice in third party processing

Question

We have a requirement to bill a customer before the vendor invoice in third party processing. The reason behind this is because the vendor send invoices only once in a month and the customer cannot wait for so long to to receive their invoice from the business.
I saw some discussions that the third party processing with shipment notification can be used however when I check the business flow in that case I see the following.
A Third-Party Sales Order is created and a purchase requisition is generated automatically 
A list of purchase requisitions to be assigned is displayed 
The assigned purchase requisitions are converted into purchase orders 
The purchase orders are approved 
A statistical goods receipt is posted 
The vendor invoice is verified and posted 
The billing is created
It clearly says that the customer billing document is only possible after entering the invoice from the vendor. Which would be the best way to fullfill this requirement?

Answer 1

Do you mean the goods issue done by the vendor to the customer in the vendor system? And this quantity will be copied into our ( where the third party order is created) system?
No, MIGO done by in our company system only. May be you might have misunderstood because of  my sentence.instead of company i mentioned as Vendor.
For Vendor this process will become as standard sales and he will do PGI. However in our system we do MIGO .
Suppose the customer ordered 10 Qty. Vendor delivered 2 Qty wouldn't the above equation Order qty- MIGO qty= Invoice qty be 10-2= 8? but we should be really billing the customer only for 2 right?
No you have mistaken my idea, it was not a formula just i was giving example.
Billing qty is nothing but it is qty copies from source document to target document.
E
Goods receipt quantity less invoiced quantity
for example order qty is =10 and vendor delivered first time 2 qty to the customer and he billed our company for 2qty and we billed the customer for 2 qty.
And next time let us assume again Vendor delivered 4 more qty and billed us 4qty.And this time when you are billing the customer in VF01 system will propose the 4qty only( total goods receipt qty 6 - already invoiced qty 2).
Hope now you understood the meaning of E(goods receipt qty less invoices qty).
And did you try your requirement as mentioned in my first reply?
thanks,
Srinu.