Getting almost nowhere with my buttondrag script

I am trying to make it so that you can drag a gui button using script ,but my script isn't working so far. I tried to make it so that when you click the button it then follows the position of your mouse from the position it started in...that didn't work.What did I do wrong? I need to detect if the mouse is staying down on the button and then how far the mouse has moved. I commented some lines in the script to show you what I'm trying to do. Any help for me. Thanks!

 using UnityEngine;
 using System.Collections;
 
 public class InventorySystem : MonoBehaviour 
 {
     //RANDOM VARIABLES
     private bool inventoryOn;
     public Texture emptyTex;
 
     //INVENTORY VARIABLES
     public GUIStyle buttonStyle;
     private int buttonSize = 70;
     private int sBB = 4;
 
     //SPACES IN THE INVENTORY
     private int cols = 4;
     private int inventorySpaces = 16;
 
     //DRAGGING THE SPACES
     private Vector2 slot;
     private bool dragging;
     private Vector2 mousePos;
 
     void Start ()
     {
         inventoryOn = false;
         dragging = false;
         mousePos = Input.mousePosition;
     }
     void Update () 
     {
         //INVENTORY ON AND OFF
         if(Input.GetKeyDown(KeyCode.Tab))
         {
             if (inventoryOn == false) 
             {
                 inventoryOn = true;
             }
             else if(inventoryOn == true)
             {
                 inventoryOn = false;
             }
         }
         Vector2 mousePos = Input.mousePosition;
     }
     void OnGUI()
     {
         InventoryLogic ();
     }
     void InventoryLogic()
     {
         int cnt = 0;
         int x = 0;
 
         //IF THE INVENTORY IS ON
         if (inventoryOn == true)
         {
             for( x = 0; x < inventorySpaces; x++)
             {
                 //STARTING POSITIONS OF THE SLOTS
                 slot = new Vector2((x % cols) * (buttonSize + sBB) , (int)x / cols * (buttonSize + sBB));
 
                 if(cnt < Player.Inventory.Count)
                 {
                     //SLOTS WITH ITEMS INSIDE OF THEM
                     if(GUI.Button(new Rect(slot.x,slot.y,buttonSize,buttonSize),Player.Inventory[cnt].icon,buttonStyle))
                     {
                         dragging = true;
                     }
                     
                 }
                 else
                 {
                     //EMPTY SLOTS
                     //CREATE THE SLOTS HERE
                     if(GUI.Button(new Rect(slot.x,slot.y,buttonSize,buttonSize),string.Empty, buttonStyle))
                     {
                         //TESTING PURPOSES SET DRAGGING TO TRUE
                         dragging = true;
                     }
                 }
                 cnt++;
             }
         }
 
         if (dragging == true) 
         {
             // IF DRAGGING IS TRUE THAN TAKE THE ORIGINAL  X AND Y VECTORS FOR THE SLOT AND + mousePos
             slot = new Vector2((x % cols) * (buttonSize + sBB) + mousePos.x , (int)x / cols * (buttonSize + sBB) + mousePos.y);
         }
         if (Input.GetMouseButtonUp (0))
         {
             //MAKE DRAGGING FALSE WHEN YOU LET GO OF THE BUTTON
             dragging = false;
         }
     }
 }