Can commercial aircraft dump luggage or cargo mid-air?
No they can't. The cargo doors are secured and will not open mid-flight. 

Cargo holds are usually pressurized so opening the door anyway would cause depressurization.

Also if the weight was too high the pilot should never have lifted off in the first place.

Then after you theoretically throw the stuff off board think about what happens after to the stuff. It will drop down hard probably on someone's roof.

Only some military cargo planes with a back hatch are designed to jettison cargo.