This is a hypothetical situation: I'm asking not because it occurred at the table but because I think different POV's about roleplaying/rules might result in different answers, which might be interesting.
Situation: a player wants to play an Aasimar, but thinks the flying is kind of cheesy/OP, so describes the character as having burn-scarred stumps instead of wings (with an appropriate backstory explaining how this happened.) The player asks the DM for nothing in exchange for giving up this ability.
Many levels later, the player finds him/herself in a desperate situation where a short burst of flight will save the party from TPK. He/she narrates that in a burst of divine energy, newly formed wings burst forth from the stumps, and the character proceeds to fly, as per RAW in SCAG.
Would you allow it? Why/why not?