Is a air inside the balloon a sample of matter?

Yes. Matter is anything that exists. On earth, it is very difficult to NOT have something that is a sample of some kind of matter. Air is a mixture of transparent gases, but it has pressure and weight and mass; thus, it is matter. Unless a container is a perfect vacuum (such as you'd find in interstellar space, or a science laboratory with complicated equipment for artificially creating one) it contains matter.