wow that sucks
first (and I hate to point this out but I know someone will) NEVER send anything through an FFL that doesn't absolutely have to go through an FFL. Yes, you'll pay extra for two shipments but FFLs can be a disaster when it comes to incoming and outgoing transfers. same for repair service btw, especially a factory repair.
second, did you call the FFL and talk to them? forget email/text msg, call them during an off hour and ask them to look really hard. it's not uncommon for FFLs to have boxes and boxes of guns coming and going and accessories piling up.
lastly if after giving the FFL some time and calling them a few times, I guess the best you can do is offer the buyer a reduced price and if he doesn't want to accept that then have them send whats left back to you. If the FFL agrees it's "missing" then I'm not sure how you can hold the buyer responsible. only option left I see is to plead his forgiveness and hope he cuts you a sympathetic deal.
good luck