When Reverse Magic is revealed, you take control of that spell. Although the Cast Spell Step is over, you are allowed to adjust the spell's choices and pay additonal mana costs if any.
With Dissolve, you take control of the spell. It now targets the original casting Mage, and you can now choose a new piece of equipment for Dissolve on that Mage. If that piece of equipment costs more than the cost has already been paid for Dissolve, you must pay the difference. If it costs less, you do not get any mana refunded.
Same thing with Reverse Magic on Force Push. If the original caster did not pay the extra mana to push through a wall, but you want to push him through a wall, you can pay the extra mana to do so. If the opposite is true, you do not get a mana refund for choosing to not push him through a wall.
As much as possible, we want things in Mage Wars to work the way you think they should work.
We're sorry that this text does not appear on the Reverse Magic card. The card was already a little text heavy! You will see this text in our coming Spell/Rules FAQ, and also in our online card database (coming very soon!).
Sorry for the confusion, and hope this helps.