Just go to the dealer and order another set of the 'buttons' that stop the buckle from dropping down to the floor. You'll need something like a soldering gun or heat up say the head of a finishing nail to melt the tab once you've snapped the two parts together to then hold it in place, but it's quick and easy. One half has a sharp point on it that goes into the center hole of the other side...then, when you melt the pointed end, it fills the hole so it can't come back apart unless you cut it off (and then would need to replace it).
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above
After installing the buttons on my seat belts I found out that the passenger belt buckle when not used was rattling a bit against the door so I installed a black Velcro strip where the buckle hits the door and that fixed it.
Just ordered a pack of the clips today when I took the loaner back ready to fit on my yet to be born i3.
The belt was already driving me nuts for two reasons. Firstly as noted by others, the buckle rests down by the floor for some unknown reason. Secondly, the driver's belt was twisted somehow. It wasn't possible by hand to get the twist back out by passing a double thickness fold through the buckle so I am guessing that either the belt had folded over when retracting into the door mechanism or it was fitted twisted from the factory. Either way pretty maddening and not the first car I've taken all the trim off to unbolt, untwist and re-torque the belt retaining bolt.
I thought about doing a clip like the above mentions when I recently found a solution that works for me. When I remove the belt, instead of letting it drop to wherever, I put it at the top of where the belt comes out, above your left shoulder. It stays there till you need to use it again.