Let's say that you have a listing with a multiple quantity of 15 and the buyer wants to buy 100
The correct way to do it is to revise your listing and change the quantity from 15 to 100 and then let the buyer purchase the quantity of 100. Then send an invoice.
If the buyer already purchased 15, then you can create a similar listing with a quantity of 85 and let the buyer purchase the quantity of 85. Then send an invoice combining both purchases (15 + 85)
It would be easier if you had been more specific in your question (listing number....)