You can't delay the I-140, but you can delay the second stage (the actual visa application) for as long as you need to. You just need to take action at least once a year, but that can be paying a fee, submitting a document, or even just logging in.
And you won't get a green card prior to immigrating, you'll get an immigrant visa. Only once you enter the US on that visa do you then get the green card.
Best of luck.