I get what you're saying, but it could just be a way for a new employer to say "we have a policy of treating people fairly. We know that some new hires are going to lose out on bonuses depending when they join, so we have a policy of compensating these new hires for this loss".

I wouldn't want to start off with a new employer by negotiating for a bonus buyout and then staying long enough to get my original bonus (and expecting the buyout anyway). I would be interested to know if anyone has done this successfully. If I thought I had the upper hand in negotiations, I'd just ask for more pay, or a signing bonus independent of my foregone bonus.

If an employer offers a bonus buyout during negotiations, that's a straight-up offer of more compensation, in the form of a signing bonus. It's hard to come up with scenarios where an employer would offer you a certain total compensation, and then revoke that offer because a different, previous company paid you a bonus. Would you expect your employer to reduce your compensation if you refinanced your mortgage, in the name of fairness, since that portion of your salary was "intended" to cover housing costs? Compensation is about estimation of value (expected to be) delivered vs costs (expected to be) incurred, in the presence of highly asymmetric information. Previous bonuses might give nice framing for certain points during the negotiation, but that's just storytelling; the real negotiation doesn't care about them, and only slightly cares about start date in most circumstances.

Say the company hiring you agreed to pay relocation expenses from NYC to SF. If they then learned you were actually already living in SF they wouldn't just give you the money.

If the company went through the approval chain and got everyone to sign off on you being worth $comp + $relo, and you're value isn't effected by not choosing to $relo (because, in this case, you're already there), it should be pretty trivial to get everyone to approve a change to call that $comp + $signing_bonus since everyone involved already agreed to spend that portion of their budget on you. The case this doesn't apply is if their assessment of your value changes because they no longer trust you as much -- which does hint at an underlying rule of "even during a negotiation, don't be a dick" -- but the failure risk here is that you're providing symmetric access to the information that you're an asshole they might not want to work with, not that you asked for the compensation to be categorized differently.

So, yeah, I definitely have never solicited a bonus buyout and then done a bait-and-switch. But I've never actually solicited a bonus buyout -- I negotiate in terms of compensation I receive, not labels on it. I've had companies offer me bonus buyouts before, sometimes quite significant, and I've always taken that as a direct indication of their assessment of my value, not as something specific to the time of year.

What you're doing sounds fair. My reading of the upthread discussion was that someone who had asked for a bonus buyout should delay their departure until they could double-dip. That would be a huge red flag to me, if I were a new employer. But if it's all just part of comp, there's no specific issue.

Of course, the new company can say "you get X signing bonus if you join this calendar year, and x/2 signing bonus if you join next calendar year, since time is of the essence".

This sounds like pretty bad, or at least risky, advice. Mind you, I think it'll work on average; but it's a dishonest and adversarial way to start a new relationship, for probably not a ton of money.