I just had a situation last week like this! The interview process with our client had been about 6 weeks (throw in the holidays, a wedding and a funeral, not a normal time frame). As with all candidates, we lock them down every time we talk to them. As I was going to make the offer to him, he came back and said he wanted $5k more per anum, PLUS financial consideration for the trial period to cover medical benefits. ALL this had been discussed before, every time we talked! To say I was surprised is a bit of an understatement.
I took a breath, then withdrew the offer to the candidate. I am authorized to speak on behalf our of client when it comes to salary negotiations, and I was not going to allow this to happen at such a late date. NOTHING CHANGED except he now had the offer. I really felt like we had been operating under bad faith with him. So, I withdrew the offer.
My client was fine with that choice. They know we look out for their best interests and anytime something like this happens, it simply will not end well. It never does.
Whenever something like this happens, I always go back and re-evaluate the situation and see if we could have done something different. I honestly don't know what we could have done different in this case. I'm glad it worked out the way it did, however, because it could have been a bigger headache if he went to work for us!