Command Headache - money system /w command blocks

It depends how you're doing it. Let's say for the sake of example that a player has just paid 1 emerald and is now expecting 16 gravel in return.

If you're using /give, it's as simple as including the amount:

/give PlayerNameHere minecraft:gravel 16

If you're putting the items in a chest or having a villager trade them or something, you can change the number of items in a stack by using its NBT data:

/... {...{id:minecraft:gravel,Count:16b...}...}

That's also how you'd test for the expected amount of currency. Sorry if this is unhelpful, I'd really need to see your commands to know how this would fit in.