The candidate could put their minimum acceptable salary in an envelope. The employer could put their maximum acceptable salary in an envelope. The two parties could agree with a third party arbiter that if the maximum from the employer is larger than the minimum from the candidate, the salary will be set to the midpoint between the two numbers, and otherwise, no deal can be reached so both walk away.

That's one way, at least, where neither party has to concede the information advantage.

In fact, I'm surprised there aren't already 37 start-ups that offer this as a service. You could have a cute little envelope logo like the MS Outlook logo and give a TED talk about how you're totally disrupting information asymmetry in recruiting, and eventually be pressured by techtopus to provide data about what candidates are asking for to perpetuate information asymmetry.