Ideally, the best way would have been to have dug it while it was still fully dormant and tranferred it to a suitably sized container until it could have been replanted. Waiting until May is possibly the worst choice of timing. Can you make arrangements with the new owners to come back and retrieve your tree in fall after it loses its leaves? Barring that, digging it ASAP and potting up until it can be replanted is an alternative, although still not great timing.

One suggestion I have is, if you are going to move it now, dig the new hole first. Then dig up the maple. And if you are going to leave it behind to get in the fall, get it in writing with the new owners. I would also suggest if you have to dig it up, go to a local nursery and get a big container to set it in rather than trying the balled and burlap route.