This (and the other suggestion by zerr) still leave the potential for someone to create multiple false 'minimum' datapoints, then target one of their coworkers to find out their salary.
Yeah, it's not at all foolproof or even great. I don't think there's really a good solution that still provides true anonymity, since the only way to prevent box-stuffing is to actually restrict people to one entry per real person (and that generally requires a more involved real-world authentication process).