Ok you can try the below approach:

Declare
  v_item inventory.item%type := :p2_item;
  v_qty number := :p2_saleqty;
  /* get the quantity available date wise */
  Cursor c_item
   is
  Select qty_in_hand, rec_date
   from inventory where item = v_item
  order by rec_date asc;
Begin
  for c in c_item loop
     if c.qty_in_hand >= v_qty then
        Update inventory
           set qty_in_hand = qty_in_hand - v_qty
          where item = v_item and rec_date = c.rec_date;
          v_qty := 0;
     else
        Update inventory
          set qty_in_hand = qty_in_hand - c.qty_in_hand
        where item = v_item and rec_date = c.rec_date;
        v_qty := v_qty - c.qty_in_hand;
     end if;
     if v_qty = 0 then
         exit;
     end if;
  end loop;
  commit;
End;

Add a receive date field in the inventory table, and then you can update the quantity in hand on behalf of this field. Below is an example:

Update item_inventory a
  set a.qty_in_hand = a.qty_in_hand - v_sale_qty
 where a.rec_date = (select min(b.rec_date) from item_inventory b
       where b.qty_in_hand > 0 and a.item = b.item)
  and a.item = v_item;

Where the v_sale_qty and v_item are the variables for the item you want to update the inventory.

This is just an example to give you an idea. You can modify more as per your requriement.

Looking around, I see that you mean to add a page-level validation which validates 2 items.

I have 2 buttons. One button requires only one field to be filled. The other requires both fields. Would it not be better for each field to validate itself, with the validation code dependant on the button pressed? If so, how do I know which button was pressed?