Is matter and mass always the same

No, because the matter of which it is made remains the same, But weight is not a good measure of how much matter he has. scientist needed to describe the amount of matter, independent of weight. That word is mass.

Matter is a slightly archaic word for something with mass, as in the conservation of matter (which must be paired with the conservation of energy to still hold true. Mass can be converted back and forth between energy, so therefore so can matter. Of course relativistic mass is conserved as it's a function of the energy of an object in that reference frame.