Resource Monopoly when player only has one of the named resource
They must give the one they have.

This is covered by the rules.

Resource Monopoly
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.