Re-trading the job offer

In many of the companies I work with, I sign candidate offer letters.  This isn’t always something a CFO does, but if possible, I prefer that I (or the CEO) am the final check on this particular business process.  One obvious reason is that I know what’s in the budget for a given position.  One less obvious one is someone with a touch of OCD will find errors – incorrect years, language from an old template that doesn’t belong in this particular letter – that someone else might not.
Another reason is that recently for somewhat junior positions, I am seeing a lot of negotiating by candidates who have received a verbal offer, accepted it, and now wants to re-trade.  It’s hard for a hiring manager to resist this.  With rare exceptions, I always do.
A candidate who has accepted verbally a salary of $100, but then when they get the letter and ask for $105, is setting him or herself up for failure.  He’s now signaled that either (a) everything is going to be a negotiation, (b) I can’t necessarily trust him to keep commitments or (c) my offer is the stalking horse for another one.  In my experience, most often it is (c), but (a) and (b) do also come into play.
I have sales managers tell me that this is just someone who is valuable in the market negotiating hard for themselves.  I don’t see it that way.  Maybe I am biased because credibility is my only product.  In my view, once the parties agree, a negotiation is done.  If someone really is a superstar, we’re going to find that out and likely make an adjustment upward anyway.  Or if they have variable comp, they’re going to crush their numbers anyway so the base salary doesn’t really matter.  Needless to say, it’s nearly impossible to adjust down.
Because I work with build stage companies, the hiring managers often are young and haven’t had the scars yet of candidates re-trading their job offers because what they really want is more money to stay where they are. My advice is to resist.