The kids were playing with the passenger front seat belt. Now it will not come out. It is fully retracted, but there is a little slack, perhaps 2" slack, enough to let it retract, but when you pull on it, you get an inch or two of outbound travel, and then the pawls grab, as if the belt were pulled out rapidly.
The part is $200 to $300, plus opening up the interior column. Any thoughts?

My other car is a 1995 E320 wagon w/ over 200k miles.

I've seen this kind of thing happen on non MB cars. For a normally operating belt, the act of retraction lifts the pawls from the reel. If the mech is artificially energized when the belt is retracted / near retracted, there isn't enough retraction left to lift the pawls.

Belts will lock a few ways, rapid extension of the belt, rapid slowing of the vehicle and ,in some cars, pretensionors that work with the air bag system.

Before you start the process make sure the car is on level ground. ( If it was not before, put on the level and try to pull the belt out again. )

Shine a light in the retractor hole, you might get lucky and see the locking pawl / bar. If you do, use a thin implement to lift the pawl then extend the belt. If this works, hold the belt out a foot or so then give it a quick tug, it should lock. Let the belt retract and try to pull it out again.

If you could not get the belt to extend at any time, try to feed as much slack by pulling hard towards the reel then slowly release and pull the belt out.

If this does not work, is the end anchor of the belt accessible? Unbolting it would give you extra slack to reset the mech.

Past that, the retractor will have to be unbolted from the vehicle and reset / replaced.

You may want to check with your dealer to see if they will cover service or replacement. Many manufacturers warranty seatbelts for the life of the vehicle.

__________________
1989 300CE

Understandably, the dealer said don't mess with a seat belt; replace it.
Faced with an dealer appointment, and a $300 part, I had a chance to put more time into it today. Creating more slack did not help. I eventually opened up the pillar (remove the fabric door seal , front and rear doors, adjacent to the pillar by pulling straight away. Next, pry up the door sill. Now you have exposed 4 clips which hold in the bottom portion of the pillar face-trim. Pop the trim piece out of those clips. There's the unit).

Disconnect the battery, as these belts have pyrotechnic firing devices.
I found a black plastic cap on the after face, and within that were a cluster of little rachety-gear thingys. Bumping them around a bit resulted in a Voila moment. It works!.

Thanks for your comments.
-Paul T