Tendering BJack would have ensured no one was going to pick him up. The low offer this year is $1.2M, so I would think any team making him an offer would have to go higher than that to get him to leave the Packers and then part with a 2nd round draft pick on top of it. I don't think he'd be worth that, so we'd have him at $1.2M, and I'm not sure I'd pay a 3rd RB that much.
I wouldn't want to rely on compensatory picks giving us a 3rd rounder for JJ. The picks are set by a formula that considers salary, playing time, post season honors with new team, etc. I would have tendered JJ the $1.2M and then either we got him back cheap or a 3rd round pick for him.