Fading all but single instance of a button

I have a menu with 4 buttons. If the user taps on one, I highlight it to show that it is selected. When the user places the item corresponding to the button in the game scene, the button fades back out.

The problem I am having is that when I have a button selected and then select another button, both buttons are highlighting. The behavior I am trying to achieve is the first button fades when the second button is pressed.

I have tried setting the selectedButton to null, which calls the FadeAll() method from Update(), but both the first and second button clicked remain highlighted. When I click on the game scene, the GameObject corresponding to the latest button press is placed, and all buttons are faded.

Here is my class that controls button fading:

 using UnityEngine;
 using System.Collections;
 
 public class Button : MonoBehaviour {
 
     public static GameObject selectedButton;
     [SerializeField] private GameObject thisButton;
     private SpriteRenderer sprite;
     private Color originalColor;
     private Button[] buttons;
     private StarDisplay starDisplay;
 
     void Start () {
         starDisplay = GameObject.FindObjectOfType<StarDisplay>();
         buttons = GameObject.FindObjectsOfType<Button>();
         sprite = GetComponent<SpriteRenderer>();
         originalColor = sprite.color;
         FadeAll();
     }
     
     void Update () {
         if (!selectedButton){
             FadeAll();
         }
     }
 
     private void FadeAll(){
         foreach (Button button in buttons){
             Color fade = sprite.color;
             fade.a = 0.5f;
             sprite.color = fade;
         }
     }
 
     void OnMouseDown(){
         //TODO error, if select one then select another, both highlight
         selectedButton = null;
         if (thisButton.GetComponent<Defender>().StarCost <= starDisplay.StarCount){
             selectedButton = thisButton;
             sprite.color = originalColor;
         } 
     }
 }

This is the relevant code from the other class that fades all buttons when one object is dropped into the game scene.

 using UnityEngine;
 using System.Collections;
 
 public class DefenderSpawner : MonoBehaviour {
 
     private Vector2 targetLocation;
     private GameObject parent;
     private StarDisplay starDisplay;
     private int defenderCost;
     void Start () {
         starDisplay = GameObject.FindObjectOfType<StarDisplay>();
         parent = GameObject.Find("Defenders");
         if (!parent){
             parent = new GameObject("Defenders");
         } 
     }
 
     void OnMouseDown(){
         targetLocation = CalculateWorldPointOfMouseClick(Input.mousePosition);
         if (Button.selectedButton){
             defenderCost = Button.selectedButton.GetComponent<Defender>().StarCost;
             if (starDisplay.UseStars(defenderCost)){
                 GameObject defender = Instantiate(Button.selectedButton, targetLocation, Quaternion.identity) as GameObject;
                 defender.transform.parent = parent.transform;
                 Button.selectedButton = null;
             } else {
                 print("Cannot afford defender");
             }
         }
     }
 

In the second class, all buttons fade when the selected Button == null. But in the first class this is not happening when I set selectedButton = null in OnMouseDown(). Any ideas for another way to go about this would be greatly appreciated!