The "free action before combat" thing is a common desire from players, but it doesn't exist in 5e. They can declare their intent to do something all they want, but initiative is required to know when it all happens.
I think the problem is that players look at their "free action" as the thing that triggers combat, when it's really just their first action during combat. Like, if you're talking to a shopkeeper and he's not giving you the discount you want, you don't get to stab him in the face and *then* roll initiative. You say you are going to stab him in the face and the GM starts combat to determine when it happens. If the shopkeeper wasn't surprised (maybe he's a veteran shopkeeper and knows a violent customer when he sees one), it's very possible that he'll be quicker on the draw and see that you're going for your dagger, etc..
But no, there isn't a "free action before combat," so there's no need to figure out how to handle the situation. Just have the players roll initiative determine if anyone is surprised.