Sweetie, there used to be an entire cow surrounding that piece of beef – every bit of it is a cut end. Butchers and meat-cutters do not perform magic, nor do they coat the roast with some substance to keep the juices in. (Nor does searing the meat keep the juices in….)

You will not cause yourself any problem at all in cutting it down and saving some for later. (Obviously you'll cook the roast for less time. And obviously you'll store the little roast for a reasonable amount of time, before you cook that, as well.)

Your other option is to not give your guests a choice, but to serve the roast now and save the ham for New Year's Eve, or vice versa.