i dont really know but he wants a item banking system so he could try this, but i know another way: one variable for each item he wants the player to store in the bank, than if he stores an item you delete it from the inventory and do the item variable+1, when he wants back his item you give him the item and do the item variable -1, example:
the player wants to store an kings sword, than you take away the kings sword and do the kings sword variable+1, when he wants back his king sword you give him the king sword and do the kings sword variable-1.
warning: this can cost you like 10 hours when you have much items, you should only do this if you really want it, if you do just make 1 item this way and (when you're sure this doposit/return for this item works) just copy and paste this like 100 times, you only have to change the item and the variable right than wich can cost you like 3 hours less.