Change the position of the float rect whenever the position of the sprite is changed.
You can do this manually or maybe encapsulate in a class to do it automatically.
e.g.
(incomplete, obviously)
class Enemy
{
sf::Sprite sprite;
sf::FloatRect hitbox;
setPosition(sf::Vector2f newPosition)
{
sprite.setPosition(newPosition);
hitbox.x = newPosition.x;
hitbox.y = newPosition.y;
}
}
Then it's just, for example:
Enemy enemy;
// ...
enemy.setPosition({ 200.f, 150.f });
Of course, if the float rectangle is not exactly the same co-ordinate, you'll also need to offset those values.