@Luis Normally the most common workflow is: 1- Sales Order. 2- Fulfillment. 3- Invoice. Are you using a custom business process/workflow where you create an Invoice before you fulfill it? (I'm curious here how do you know you have the inventory before fulfilling the order?)

Yes, our custom business process is to make sure that the customer is invoiced first (sometimes paid first) before we fulfill the inventory. Tracking the inventory though is not problem for us.

Well, if your Invoice has a Created From = The Sales Order you want to fulfill, then it is possible to create a button to transform the Sales Order to Item Fulfillment but I don't think it would be possible with a Workflow specially considering the quantity match requirement, it may need scripting.