There are a couple of posts on brickboard that may describe a similar
problem.  See

http://www.brickboard.com/AWD/index.htm?id=729140

I have a 97 850 wagon, and the past two times when I have flopped the back seats
down, the middle seat belt which is drawn out when you put the wider seat down,
refuses to release and let you pull the seat back up. I have had to unbolt the
belt from the floor to get the seat up, and then reattach it. But the same thing
happened the next time I flopped the seat down. At present I drive around with
the rear middle seat belt disconnected. Is there a fix other than replacing the
seat belt mechanism? And, if I have to replace it, which model in a junkyard
should I take one from so the same thing won't happen again? The retraction
mechanism is in the back of the seat, and is there a horror story connected with
changing it out?

And a reply:

Look at the belts that you don't have a problem with. Note the plastic button
that prevents the belt from sucking in too far. Now look at the belt that sucks
itself in. Bet the stop button is missing. These are available at the dealer for
a dollar or 2.

The seat belt retractor has probably failed.
All Volvo seat belts are under factory warranty
for 5 years unlimited mileage.
The dealer will replace the belt free of charge.

BD
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