I had a problem with the same figure. The set I purchased was advertised as MIB, but arrived with two of the figures badly damaged, including the figure you received damaged. (Apparently, this is a stress point on the weapon that many of us have had problems with). But mine arrived with the grenade extension completely snapped off. This was before K & C was using die cut foam that you can completely recess the figure into. At the time they were primarily using the "egg crate" style, convoluted foam. A lot of these older sets were subjected to the figures banging into one another in transit if they were not carefully packed. Even then, the soldiers don't have to move too far to clang into each other. I also noticed in some of my older sets that there were variations in the convoluted foam ridge heights (perhaps intentional?). In a few sets the foam peaks were very short, allowing for more free movement of the figures in the box than had the foam had taller ridges. Perhaps a bit of over analysis here, but I thought I'd share my experience with you as well as my observations of the older style foam. In fact, I have sold a few older type K & C multi-figure sets that had shorter peaked foam, and instead of taking the risk of having the figures bounce around and hit each other (which typically can be solved with three strong rubber bands around the closed box) I left one figure in the box and packed the others in their own small boxes, carefully supporting extended appendages and weapons/accessories with packing peanut fragments, and then carefully wrapping the individual figures in tissue. I know, these may seem like extreme measures, but better this than to have to go through what you, I and others have had the displeasure to experience. Again, I have only gone to this extreme a few times and that was with the eggcrate type foam in instances where the ridges seemed more shallow than was typical. Thankfully, as far as I know, K & C no longer uses this style foam. I believe they only use the die cut style now that has a slot in the foam the same shape of the figure that it fits firmly into. The depth of the recess is deep enough to cover the whole figure and is much more effective than the eggcrate foam.
The good news is, I think your figure can be repaired. There are many skilled folks here on the forum who would probably be more than happy to assist. I would just very carefully bend it back straight. The paint will be cracked at the stress point (most likely all the way around), but it can be fixed. Hopefully, it won't snap on you in the process. You may wish to hail the services of The Kilted One, the Vampire Field Surgeon known to us all as KV. He buys broken stuff just so he can restore it to like new or better condition! One final observation of mine, K & C weapons are very soft, so they are more likely to get bent, but they are also more often repairable than say harder materials like fine pewter or alloy metal compounds that would've just shattered rather than absorb the shock and/or contact. I hope this helps! This happens to pretty much everyone from time to time. And don't let the guy calling you a liar get you down. He's just a creep, whoever he is. Just get a medic for your soldier, and leave that seller in your rear view mirror; he's just a bump in the road.Don't let him spoil your enjoyment of your collection!

Joe