I think the problem with this is the contract.
The rams are a great example of how this can work. Trade a high pick for a one year rental and then let them walk, like the Watkins situation. The difference is a high comp pick comes into play when letting these players walk on an expiring contract. That really softens the blow of the compensation.
While McCaffrey wouldn't be a big cap hit this year, and we could walk away after, no comp picks would come into play with the loss if we cut him after this year. Are we really going to burn a #1 for a half year rental with no compensation coming back? I see little chance we would move forward with his salary.