Need help with my player attack script

Hey guys.

I have a PlayerAttack with bullets. Currently if I collect a bullet prefab, in the InventoryManager-Inspector the bullet float rises up.
If the bullet in InventoryManager greater then 0 is and I press the keycode f then it shoots. So far so good.
Now I want that in the InventoryManager-Inspector the bullet float should fall down if I shoot once. I don’t know how to do that.

PlayerControllerScript with bulletattack on line 73 and 89:

using UnityEngine;
using System.Collections;

public class PlayerControllerScript : MonoBehaviour 
{
	public InventoryManager inventoryManager;

	public float maxSpeed = 5f; 
	public bool facingRight = true;
	
	Animator anim;
	
	public bool grounded = false;			
	public Transform groundCheck;
	float groundRadius = 0.2f;
	public LayerMask whatIsGround;  
	public float jumpForce = 650f;

	HealthController healthController;

	//Player Attack
	public Rigidbody2D bulletPrefab;
	float attackRate = .5f;
	float coolDown;
	
	// Use this for initialization
	void Start () 
	{
		anim = GetComponent<Animator>();
		healthController = GetComponent<HealthController> ();
	}
	
	// Update is called once per frame
	void FixedUpdate () 
	{
		if(healthController.currentHealth > 0)
		{
		grounded = Physics2D.OverlapCircle (groundCheck.position, groundRadius, whatIsGround); //Here we are in the ground
		anim.SetBool("Ground", grounded); 
		
		anim.SetFloat ("vSpeed", rigidbody2D.velocity.y); 
		
		
		//if (!grounded)return;  
		
		float move = Input.GetAxis ("Horizontal");
		
		anim.SetFloat ("Speed", Mathf.Abs (move));
		
		rigidbody2D.velocity = new Vector2(move * maxSpeed, rigidbody2D.velocity.y);
		
		if(move > 0 &&!facingRight)
			Flip ();
		else if (move < 0 && facingRight)
			Flip ();
		}
		else
		{
			gameObject.renderer.enabled = false;
		}
	}
	
	void Update()
	{
		if(grounded && Input.GetKeyDown(KeyCode.Space)) 
		{
			anim.SetBool("Ground", false); 
			rigidbody2D.AddForce(new Vector2(0, jumpForce)); 
		}

		if(Time.time >= coolDown)
		{
			if(Input.GetKeyDown(KeyCode.F) && inventoryManager.bullet > 0)
			{
				BulletAttack();
				inventoryManager.bullet = -1;
			}
		}
	}
	
	void Flip()
	{
		facingRight = !facingRight;
		Vector3 theScale = transform.localScale;
		theScale.x *= -1;
		transform.localScale = theScale;
	}

	void BulletAttack()
	{
		if(facingRight)
		{
			Rigidbody2D bPrefab=	Instantiate (bulletPrefab, transform.position, Quaternion.identity) as Rigidbody2D;
			bPrefab.rigidbody2D.AddForce (Vector3.right * 500);
			coolDown = Time.time + attackRate;
		}
		else
		{
			Rigidbody2D bPrefab=	Instantiate (bulletPrefab, transform.position, Quaternion.identity) as Rigidbody2D;
			bPrefab.rigidbody2D.AddForce (-Vector3.right * 500);
			coolDown = Time.time + attackRate;
		}
	}
}

InventoryManager:

using UnityEngine;
using System.Collections;

public class InventoryManager : MonoBehaviour {

	static public Inventory inventory;
	
	public float coin = 0;
	public float bullet = 0;

	public float initHealth = 5;
	public float initLifePoint = 3;
	public bool initValues = false;

	private string healthText = "Health";
	private string lifePointsText = "LifePoint";
	private string maxHealthText = "MaxHealth";

	void Awake()
	{

		if (inventory == null)
		{
			inventory = (Inventory) ScriptableObject.CreateInstance (typeof(Inventory));
		}

		if (initValues)
		{
			inventory.Clear();
			inventory.SetItems(healthText, initHealth);
			inventory.SetItems(maxHealthText, initHealth);
			inventory.SetItems(lifePointsText, initLifePoint);
		}
	}

	void Update()
	{
		coin = inventory.GetItems("coin");
		bullet = inventory.GetItems ("bullet");
	}
}

“…Now I want that in the InventoryManager-Inspector the bullet float should fall down if I shoot once.”

I take that to mean you want bullet var to decrease. You already do this:

 BulletAttack();
inventoryManager.bullet = -1;

but it sounds like you meant for that last line to be

inventoryManager.bullet--;

(what you have now is a hard set to negative one)