I agree with fluttermoth. The risk is that the lone angelfish when moved in with the discus could cause problems for the discus. It is possible that the angelfish will see itself as master of their tank.
Another thought...is there any chance the store will take the lone angelfish? This would be the best solution, re-homing it.
This is exactly what happens with angelfish if a pair forms. In very large tanks, and with a larger initial group (no less than five angelfish should be acquired at the first, and together), the other angelfish may manage, as the parents will have more fish to keep their attention. But spawning angelfish can be nasty to any other fish they take a dislike to, or see as threatening.
And, welcome to TFF.
Byron.