Something similar happened to my wife, when departing Singapore earlier this year on Qantas Business.
At Singapore check-in she was offered a 75% fare refund and a $400 voucher due to a non-reclining seat for an overnight flight to Brisbane. This offer was reconfirmed by a staff member at the gate.
Of course in true Qantas style, they then refused to honour this upon her return to Australia. All they would provide is the $400 voucher.
We made multiple attemnpts to contact Qantas customer care to resolve, but they refused to honur their original offer.
Once we mentioned they were breaking a 'contract' as there was offer and acceptance, they would no longer repsond to any of my wife's messages. Maybe once we asked to speak with the legal department they are trained to never engage with the customer again.
Anyway, my wife now refuses to every fly Qantas again, so no great loss