Should I show the "Save" button before the required state is achieved

In our web application, to save a transaction (such as an invoice) the user must select the contact's name and add at least one item to the grid.

Presently, the save button shows before these actions are complete. If they click the save button before they enter the required details they will get a message to add those details.

My concern is that having the save button shows before the required state, suggests to the user that they can actually save something.

The other solution we are considering is to only show the save button if the user has entered the required details.

What is the better solution?

Note, the application auto saves the transaction as a draft. Save though commits the transaction and can have impact on other areas.