Does the opponent have to give you the selected resource ONLY if they have 2, or do they have to give the selected resource EVEN if they have only 1?
They must give the one they have.
This is covered by the rules.
Name a resource. Each player must give you 2 of that type of resource if they have them.
You may name any of the 5 resources (ore, grain, wool, lumber, or brick). Each player must give you 2 of the named resource if he has them. If a player only has 1 of that resource, he only gives you 1.
It's basically a version of the original game's Resource Monopoly (which forced other players to give the user all of their cards of the named resource) that has been capped to two cards.