Air India — extra wheelchair charges at bahrain airport

My father s m mohsin was coming back from bahrain to new delhi by ai 940 on 14/12/2019. I had already paid the ticket amount (With wheelchair facility on whole journey) on 20/11/19. Booking reference no is hlz5t. The ticket purchased was for patna to bahrain and back. While travelling in delhi air india office did not charge any feees for wheelchair. But on 14/12/19 when he was coming back, airindia in bahrain charged 29.00 bd for wheelchair. Please refund the excess amount charged.