This can be done with eventing. You need 2 variables for every item you want.
For this example I'll uses variables 7-12, with items Potion, Ether & bombs.
These are the variables
Variable 7 = Potion Before
Variable 8 = Potion After
Variable 9 = Ether Before
Variable 10 = Ether After
Variable 11 = Bombs Before
Variable 12 = Bombs After.
On the event that gives the quest. After the quest is given and accepted (ie the player says yes)
Go to control variable and set Variables 7,9, & 11. To game data number of items in inventory.
ie variable 7 is set to the number of potions the party has, and variable 9 is set to the number of ethers the party has.
On the event where you check if the quest is completed or not.
Under where the quest failed check is run and returns true.
Set variables 8,10 & 12 to game data number of items in inventory.
ie variable 8 is set to the number of potions the party has, and variable 10 is set to the number of ethers the party has.
Now then set up variables 8,10 & 12 to do some math.
Variable 8 is going to be set to subtract varable 7.
Variable 10 is going to be set to subtract varable 9.
Variable 12 is going to be set to subtract varable 11.
Basically make it where the after variables subtract the before varables.
Now open up the change item command.
In the change item command pick each item, ie pick the potion and set it to decrease by the after varable.
Ie potion should be the item you pick, select decrease by variable, and the variable you pick is varisble 8.
Now do this for all the items you have and done.
For more info on how to use variables try this
video.