To be more clear (and I came back to edit for more clarity and saw your reply that paralleled my ambiguous construction) it's about the VC (you) not backstabbing your coinvestor. IF (and it isn't clear from my reading) the company didn't have signed papers with your first coinvestor, they are well within rights to drop the investor. But you have a responsibility to not abuse the trust the coinvestor gave you. If the company, gets a better whole offer, good on them. But they can't chisel apart the first offer.