Because Horford holds a player option, I don't think he can be traded unless he excersies his option and agrees to a sign and trade after July 1st.
I could be wrong about that but I think that's how player options work.
Hmm, not sure about that. I thought that as soon as he accepted it he could be traded as a regular contract, not a sign-and-trade.
Either way, the timing and the agreement by Horford shouldn't be a deal breaker, as you would have to get Horford to buy in to the trade regardless; otherwise, he would just opt out and try to restructure or go elsewhere.