As a recruiter, have you ever worked with a candidate who, post offer, negotiated for a pay decrease? Was he/she successful?

The question is confusing by that explanation. Why would a candidate ever want a lower package unless it is made up for in another way? Every negotiation should involve weighting the benefits, opportunity, and salary of the role.  Meaning I have experiences with candidates asking for half time/ short term contracts, work from home, etc that obviously changes the weighting of the package. It is also common to have candidates negotiate for lower packages that offer higher benefits like housing, stock, children's education, etc. Rarely I've worked with candidates that the client wanted to offer yet the candidate lacked confidence in his/her ability to do the job. They requested a lower position/ less responsibility and accordingly a lower package. In most cases, clients will reconsider the candidate as it can be difficult to restructure a role in that way. Any attempt to request a lower package must be driven by some other intrinsic or extrinsic benefits to the role and package. Otherwise, as a recruiter I would think the candidate is just playing with me/ the client.

Give him/her what he wants, make him/her happy if you want to hire him, and if lower package makes him happier, well, what can I say. People are weird sometimes :)

No I have not, and anyone trying it would most certainly be successful. Unless it's a part of something more - like getting more vacation or more flexibility. I have seen people not be successful in getting such a package through.

The only reason why I can imagine how this scenario would come to be, would be if there was some tax issue with the level of income that would eventually cost the candidate more money in the long run. But if it was, then you give the candidate what they want and make them happy and hopefully they come back to you when they are in the market for a new position.