Okay, this has been bugging me lately, so, Foul Play does higher damage depending on the other Pokemon's attack. So is the user's attack effect the damage, or is it's 95 base indefinite?
It's a 95 base power move, that is definite.
The target's attack stat, and not the user's, is used in damage calculation. The target's stat boosts and any abilities that affect the Attack stat are incorporated into the calculation. The user's items affect the damage calculation (i.e. Choice Band, Muscle Band, etc). If the user has STAB (Dark type users), it is factored in. If the user is burned, it is factored in. If the user has an ability that does not affect the Attack stat but does affect damage output, such as Adaptability, it is factored in.