Ferrari

I recently took 2 trips with MH, the first I travelled with an extra 12kg and was charge 180 USD, not a big deal as I was aware, although the kilo rate is a rip.
The second trip was the same route and I only had a carry on as the trip was shorter, so I asked for a refund at check in, the girl looked at me strange, I went on to explain, that last time i flew you charge me for an extra 12kg, and this time i have an allowance of 30kg and i'm not using it, so can you please give me back the money at the same rate.

Of course I didn't get it i was just be a jerk, but made me think, do we have a legal right to it as the ticket clearly includes an allowance, and if you don't use why should it not be refunded.

cepheid

No, you don't have a legal right to a refund. The allowance isn't itemized separately, it's included in the ticket price. There is nothing to refund. It's also in the contract of carriage, to which you agreed when you bought the tickets.

So, while it's certainly a cute idea, you have no legal recourse to it.

Of course, many airlines have implemented your idea, just in reverse... they've removed the free bag allowance entirely, so if you check a bag, you have to pay for it. If you don't check a bag, you don't pay for it. So, you'd still not get any refund, you'd just not pay. Since many airlines have already reduced the free bag allowance even on international flights, your wish might just very well come true sooner rather than later... just not quite in the way you'd hoped.