Best guess is some sort of Alternanthera.
(As usual) I recommend growing a couple of stems emerse, to do ID by flower. Alternantheta is quite easy to grow emerse and will usually flower very young. The flowers will appear as quite small, usually white or faint pink "snow-balls" where leaf meet stem.
A standard flower-pot and standard flower-pot soil will do. Plant some short cuttings and water well. Cut bottom off a big, tall Cola-bottle and place it over the plant - the screw-lid is used to ventilate at least once a day (about 10 min.). After some time (a week, maybe) you can likely leave screw-lid off.
Place "arrangement" in a well lighted place, but NOT in direct sun at any time.