When a player is "waived/injured" during the preseason he no longer counts against the team's 80-man roster limit. This applies only to players who have less than 4 years of NFL service and only during the preseason. If it's a player like Vernon Carey who another team would claim off waivers even while injured, the Dolphins would place him directly on IR and lose a roster spot. There's no additional financial considerations involved either way (you're confusing this with an injury settlement). Got it?