Can you stop casting a spell?
Your friend is right, though he should let you do it anyway outside of a tournament.



You cannot be forced to produce mana to pay for a spell.


  117.3c Activating mana abilities is not mandatory, even if paying a cost is.


And if a spell becomes uncastable, its casting is undone.


  601.2 [...] If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, “Handling Illegal Actions”). [...]


So it would seem that you can choose to abort the casting of any spell by not producing the mana required to cast it (assuming your mana pool doesn't already have the necessary mana in it).

BUT

When this came up on Wizards's Rules Q&amp;A board recently, it was declared[1] that the intention of the rule is to handle the situation where the casting of a spell becomes illegal after it has started being cast (and possibly other situations), but it's not meant to be used to voluntarily rollback the casting of a spell. While the rule can be used to avoid casting a spell at all, you are committed to completely cast a spell you've decided to cast.

So, deciding to abort casting your spell wouldn't be legal in a tournament, and my friends wouldn't allow and such douchebag rule lawyering in casual. (They could very well allow me to rollback the action if I asked, but not because of 601.2.)




By general consensus among the rules-saavy, by bimmerbot (a very level-headed level 2 judge), and confirmed by Natedogg (the only person on that board capable of making official rulings).