Advice on how to pull this off right.

[Sir cryosin]

So I DM for a party of 3 PC's with 2 npc's. The PC's are a male human Eldritch Knight lv4 great sword wielder, male human life cleric lv1/wizard lv3( forgot what wiz), female Fanalis totem barbarian lv4. The NPC's the party got to pick there class and sud race female human kensei monk with a katana lv3 , female human new favored soul sorcerer lv3. there twins I had the monk to be killed off when her and the party trying to get her parents body's back from the Oni. But the party's wizard/cleric kept healing her. Anyway getting off topic.

So the party is back in there little village and just got done leveling up. A I was thinking of having a slaver Adventure stop by this little village. The slaver is a wizard that has 4 slaves he uses as his other party members for a Adventures group. The slaves are a male Goliath with a thunder cannon from the new artificer class, female wood elf cleric, male Fanalis shield fighter, female human duel dagger rogue and firebolt cantrip because reasons. Yes owning slaves is a thing in this world some countries don't allow there citizens to own slaves but slave own can travel with there slaves. But the country the PC party is living in does allow slaves and use them a lot. So why im introducing this slaver to the party is to percent the party to a situation where they might be able to fight these guys off or ends up captured and thrown into slavery and being transported and somewhere along the way while they're are being transported to be sold they get an opportunity to Escape. And the reason for this is to get the party out into the world and explore. Help me pull this off so the party don't feel like all choices were taken away from them. I want to have the PCS feel like they're not in control but at the same time in control. That makes any sense to y'all.

[Zarquon Illius]

If the PCs generally act good and moralistic, you could also have a business card(or something more thematically appropriate) on the slaver which implies he is part of a slavers guild, headquartered in some other city.Also point out that the slaves would be in their debt if they free them.If you are set on them being captured, give the opponents a magic item of some kind.
If they are captured, then have them wake up on the road to somewhere, with some other slaves, who could provide a strategic advantage when trying to fight or break out.
If that doesn't work, you could just make an NPC/gossip talk loudly about some plot hooks in faraway lands.It may seem like railroading though.

Lastly, if nothing works, wait till they are level 5 and way more powerful.Then you can justify more powerful enemies taking notice of them.

[MBControl]

Have one of the slaves slip them a note, not asking for help exactly, but simply leading them to a second location, possibly the slavers home. The party will chose to follow up on this mysterious clue. They'll find out the slaver kidnaps and tortures etc., and will probably hunt him down.