Last I looked, they only had $10m in cap space for 2019. A team (I'm definitely not suggesting us) with space in 2019 could front load the contract to ensure that the Steelers can't afford it. If they decided to match, they'd have to cut players or renegotiate existing contracts to fit him in. This, of course, is as of today, as we don't know yet what the real cap will be in 2019.
The problem is that if he's hit with the transition tag, a team will have to give up draft picks to sign him. Which a team could easily choose not to do, as he'll likely be the most expensive player this class. Which means Le'Veon might sit out another year.