Justin Bieber's Bugatti from Birdman ... Just a Loaner!

Birdman is not as generous as Justin Bieber would have you believe ... the car Biebs posed with on Instagram was NOT a gift -- it's just a loaner from Birdman.

Biebs posted the photo on Instagram Monday with the caption, "Uncle Stunna luv. My first Bugatti ♛ #generosity"

It makes it seem like Birdman bought the car for Bieber, but we've learned that's just not case. Birdman told Biebs he's welcome to drive the car -- reportedly worth nearly $2 million -- whenever he's in Miami.

Here's the thing ... when you lend Bieber a car ... what's the worst that can happen?