I was playing in a friend's in a 5e session a few weeks ago, and we encountered a young green dragon, I was absolutely convinced it would do fire damage, but I was very much mistaken to find it was poison.
Has there ever been a time when green dragons do fire damage or have I just not been getting enough sleep recently?
Various fantasy artwork, both old and recent, has depicted a green dragon who breathes fire. This is generally because in some myths, including those which inspired D&D's conception of the creature, dragons are imagined as reptilian, who are often green; and in some myths and stories, including The Hobbit, dragons breathe fire. More recently, Tohru, the green dragon in Miss Kobayashi's Dragon Maid (and, it is implied, who is named after Tolkien), breathes fire.
However, in D&D, the green dragon has always breathed poison gas (or corrosive acid gas, in later editions), but never fire. This is been true since the original D&D (1974).
D&D creator Gary Gygax describes the story how he invented D&D's dragons in The Slayer's Guide to Dragons. First, there was what we now call the red dragon, which breathes fire as per The Hobbit. (Gygax does not specifically credit The Hobbit for this in D&D, though it is noted as such in D&D's precursor Chainmail, which is now known to have taken many of its fantasy elements from a 1970 fan-made Middle Earth wargame by Leonard Patt, and the fire-breathing dragon in that is inspired by the dragon in The Hobbit.)
Next, looking to the original Norse myth that inspired The Hobbit (perhaps to fend off claims of plagiarasing Tolkien), Gygax noticed the dragon Fafnir in the Saga of the Volsungs, who breathes or exudes poison, rather than fire. In the original Dungeons & Dragons, this inspired the green dragon, who breathes a 5" by 4" cloud of chlorine gas.
Gygax actually says he invented the new chromatic dragon types to create more variety for his players, who could otherwise reliably outfit themselves with fire-proofing before any dragon fight. In other words, not only were you surprised that a green dragon did not breathe fire, but so was the first gaming group to ever fight a green dragon in D&D.
It's entirely plausible that somewhere in the myriad of official and third-party D&D books there's a spell or variant that lets a dragon use another dragon's breath weapon. A potion of dragonbreath which lets a player character breathe fire could be used by a green dragon, for example.
I faintly recall an old Nodwick comic where a village asks the heroes to fight a green dragon, so they show up equipped with a Green Orb of Dragonkind. One unfortunate encounter later, they learn that it was actually a red dragon, and a rare genetic trait causes everyone in that village to be red-green colour-blind.
Just in D&D, or in general?
I remember first edition green dragons spitting chlorine gas (i.e. poison). That's the norm in D&D: Each dragon color gets an element associated with it, and green's element happens to be poison. Their breath weapon is poison.
You asked whether green dragons have ever dealt fire damage, though, and you didn't restrict it to D&D.
D&D dragons can use magic. 5e lists this as an optional variant; see "Variant: Dragons as Innate Spellcasters", Monster Manual p. 88. Bottom line: Green Dragons are perfectly capable of dealing fire damage by casting Fireball, Scorching Ray, etc.
A DM can make up whatever monster he or she likes. Maybe your DM is fed up with players who have read the Monster Manual and wants to keep you on your toes, or maybe they just like the flavor of firebreathing green dragons. You might have met a homebrew firebreathing green dragon.
Outside of D&D, but still within the realm of fiction, firebreathing green dragons are absolutely a thing. For example, "Pete's Dragon" (1977) was released the same year as the first edition Monster Manual and features a green-scaled, pink-haired, firebreathing dragon.
In the realm of nonfiction, the answer is no. There's no evidence showing that green dragons are real, let alone that one of them has ever caused fire damage.
If I understand your question correctly, you are asking whether, as a matter of D&D canon, green dragons have ever dealt fire rather than poison damage with their breath weapons.
Strictly speaking, before 3e, the rules did not really include typed energy damage, i.e., "poison damage" versus "fire damage," "lightning damage," etc. There was simply damage, and the in-narrative source of the damage might sometimes permit game-mechanical differences in how the damage interacted with the world. For example, the 2e Monstrous Manual merely described a green dragon's breath weapon as "a cloud of poisonous chlorine gas." Its damage was listed separately in a table. (For comparison, the 2e Player's Handbook described the fireball spell as dealing "damage," not "fire damage.")
With 3e's introduction of typed damage, green dragons dealt "poison damage" with their breath weapons. That has remained true through 5e.
Even in older editions, dragons were spellcasters. In the 2e Monstrous Manual, the DM was instructed to "randomly determine which spells any particular dragon knows" -- which could and not infrequently did mean a green dragon ended up knowing fire-based spells like fireball.
Finally, in the 5e Adventurer's League epic module Reclamation of Phlan, it turns out that the green dragon Vorgansharax
has been possessed by Tyranthraxus, an evil spirit and infamous Forgotten Realms villain, also known as "the Flamed One." When Tyranthraxus manifests in combat with the PCs, Vorgansharax can use a legendary action to wreath himself in flames that deal fire damage to anyone within a 15 feet of him.
