When he didn't get the pay raise he was expecting, he had to take another job to 'pick up the slack'. [=to make up for the money he was not making]
The manager has to 'take up the slack' when employees don't do their jobs correctly.
We left some 'slack' in the budget so that we can hire more people if we have to.
(..) Get away from the sides. Look what they're doing. They don't want us. They want the fish. Listen up. Sean, you take the tiller, we'll get the line. Hannah, hold the slack. Sean, you wanna take us hard starboard? What? That way. Come on. - Okay. (..)2008 Journey to the Center of the Earth