After looking at The Times of London online style guide, dictionaries Merriam Webster, Cobalt and Oxford, and other various English language reference sources, there seems to be a 50:50 split in how to spell "buyback". Some use "the company engaged in a share buyback …", while others use "the company engaged in a share buy-back". Confused? Well, the following may explain the correct spelling rule, although it may be confusing to try and figure out in which circumstances each spelling should be applied. Good luck!

buyback (n), buy-back (adj), buy back (v)

The following word, "buy-in", does not appear to follow the same spelling guideline:

buy-in (n, adj), buy in (v)

So, wouldn't it be "buy-back (n, adj)"?