Why did Naruto need to summon his Shadow Clones during his fight with Pain? While fighting Pain, Naruto used his Shadow Clones to store his Sage Mode chakra. Since the clones were at Mount Myoboku, he needed to summon them with a scroll.

A) Why didn't he just release the jutsu to get back the chakra in the clone?

B) Why did he need a scroll? Couldn't he have summoned him with just hand signs?
Answer A: The one you are saying "releasing" the jutsu with the clone isn't possible. What Naruto trying to do is to release the Reverse Summoning which is a Space Time Ninjustsu not the Kage Bunshin.

It is said in the wiki that:

"all summoning-based techniques are space–time manipulation related, as they warp the targets through a dimensional void to the summoner's location. It appears that each specific space-time technique has its own unique dimensional void that no other technique can access." 

That means you can't release technique from another (void) dimension wherein the clones are like sealed in a container.

Now to release the reverse summon, well that answers B:
You need the signed contract(scroll) which serves as the passage way in order for the user (like Naruto) to release the Reversed Summoning then release the Kage Bunshin.