I would do the following
Ask the player for specifically what preparations they are taking before doing this (this may add pentalies or bouns to any roll example a player may strip naked to rid themselves of any noise coming from items bouncing around. As a player I use this tactic often or aleast down to my under clothes).
Ask the player specially how they are going to approach the giant and steal the bag of coins.
Weight this answers against how deeply the giant is sleeping, the chances for random events such as the giant's wife coming back or the giant having a bad dream.
More or less I would ask for spot and sneak checks every 5 foot with bonuses or penalties based on the last 5 feet. Such as a good spot check may reveal there is a creaky board ahead or a bad one would be kicking a pebble which causes the giant to stir a bit.
Then I would ask for a Dexterity or similar check for the actual act of lifting the bag.
If the Giant awakes, and the thief is close enough then I would rule the thief would get a attack and if it hits would do max damage. Also depending on how deep a sleep the giant was I would assign penalties to his init/attack rolls until he woke up.
As the DM that taught me told me "..Nothing is impossible just assign the right modifiers.."