Since it's unlikely you'll be able to get much use from the mana as compared to when you cast Rewind from M13, how about this instead?

[U]
Counter target spell. It's controller adds mana to his or her mana pool.

That way it becomes like Remand. If you play it at the right moment, they might not be able to use the mana. Play it at the wrong time, and they will.

Untapping four lands was scary overpowered. This seems fairer. And a sigle mana hard counter with no restrictions but a minor drawback? Far too good again. I could see it at though.

It's a common. I'm ok with it being a little bit underpowered. Its a simple card that plays up the colorless theme of the set. It'll still get play in limited as it is.

Perhaps we could bump this up to adding mana?

The problem with it adding only colorless mana to your mana pool is that when you cast it on your opponent's turn, you're highly unlikely to have another instant speed spell or ability to make use of that mana. For spells, there's only the cycle of instant speed ones that cost hybrid colorless mana, like Buckshot. Holding onto this spell just to try and fire off one of those spells on the same turn doesn't seem efficient. You have to work too hard, when building the deck and when playing, just to get that added benefit.

If you make the opponent gain the mana, it creates interesting decisions for the player. 'If I cast it, will he be able to use it towards a different spell? Do I counter a 1 mana spell, knowing it could ramp him for the turn?' This is why Swan Song, Swords to Plowshares, and Path to Exile are popular cards.

There are many activated abilities that use colorless mana. Part of the point of the card is finding a way to use the mana.

The problem with doing the inverse is that it has exactly the opposite effect. The spell becomes too powerful because your opponent probably isn't prepared for it. It just becomes a hyper efficient counter.

Look at it this way: Currently the card asks you to find a way to use the mana. If you set things up correctly, this card is great, otherwise it's just a mediocre counter.

If we reverse it, the card forces your opponent to use the mana. Unlike the first version, you don't have to set anything up at all, but it punishes your opponent for not being prepared. It becomes an incredibly powerful counter that sometimes is just a good counter.