@doppelgreener's answer is correct in all particulars; no roll is final until both rolls are final. But while you can repeatedly volley Fate Points back and forth, often times you'll see a GM locking in a number for a player to hit in advance, not because they're getting the rules wrong but out of concern for pacing and flow. Here's why you as GM might want to do that, too.
1) Mook or Bad Guy?
In Fate Accelerated, lots of the opposition you fight can just be "mooks" - characters which roll overall at a specific quality, usually Mediocre (+0) or Average (+1), and which have "strengths" and "weaknesses" at +2/-2 from their roll. These don't have to correspond to player approaches, but can be generic things, like
Rampaging Service Drone -- Mediocre (+0) Mook
Aspects: Logic Virus Virus Logic, Sparks Everywhere
Good At: Moving Unpredictably (+2)
Bad At: Doing The Same Thing For Ten Seconds Straight (-2)
I'm assuming your giving the drone an approach to defend with was just for the sake of example, and it wasn't actually a fully-statted bad guy? Because the degree to which you dramatically struggle should correspond to how important in the plot something actually is. And mooks aren't that important.
2) One-Pass Opposition: Setting the Obstacle with Mooks
The language "set the obstacle" comes from Burning Wheel, but the principle is the same: as the GM, regardless of whether you're acting or defending, lock in your roll first to let your player know what they need to hit. This means you burn any Fate Points or free invokes you want, either to reroll or to pump the end value, before your player rolls.
Mostly this is for convenience: you're going to be running an awful lot of mooks, and every time you go back and forth you make the conflict longer. But it also fits the nature of mooks in that they can make a dramatic appearance, but they don't really take dramatic action.
But what if somebody lands a real corker of a hit that would clear the table of mooks and you still have points left in the scene pool? Don't spend them, even then. Let your player have a moment of glory in good luck or good planning.
3) Now I Have The Upper Hand: Dramatic Action with Bad Guys
It's important to remember that taking an action in a conflict doesn't mean taking a single discrete action - one punch, one shot, one lunge. So after the dice hit the table, you don't have to somehow work everything you spend Fate Points on into one continuous action.
And when both sets of dice hit the table and you're seesawing back and forth over how hard a hit is, or if it connects at all, you're narrating how the Fate Points (or free invokes) come into play every time. This can easily produce the kind of dramatic struggle back and forth that you want out of a confrontation with an actual full-character-sheet bad guy.