Adding Int's from different and the same script

Questions & Answers
1

After looking for half an hour it seems like there is no simple explanation for what I’ve been doing wrong. I’m trying to make a purchasing system but it keeps saying I can’t use + or -.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Purchasing : MonoBehaviour {

	public int Money = 500;

	Inventory Inventory = Inventory.GetComponent<Inventory>();

	

	public void Marine(){
		Money = Money - 100;
		Debug.Log ("-100 Money");
		Inventory.Marine = Marine + 1;
		Debug.Log("+1 Marine.");

			if (Money < 0) {
				Money = Money + 100;
				Debug.Log ("Not Enough Money!");
				Inventory.Marine = Marine - 1;
				Debug.Log ("Marine RESINDED.");
		}
	}

	public void Assault_Rifle(){
		Money = Money - 100;
		Debug.Log ("-100 Money");
		Inventory.Assault_Rifle.Add(1);
		Debug.Log ("+1 Assault Rifle");
			if(Money < 0) {
				Money + 100;
				Debug.Log("Not Enough Money!");
				Inventory.Assault_Rifle = Assault_Rifle - 1;
				Debug.Log ("Assault Rifle RESINDED.");
			}
		}

That is the first script the main one the second one is here.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Inventory : MonoBehaviour {

	public float Pistol = 0;
	public float Assault_Rifle = 0;
	public float Marine = 0;


	void Start () {
		
	}
	
	// Update is called once per frame
	void Update () {
		
	}

At the end I changed it to a float thinking it might work better using floats.

2

I’ve got the main integer problem fixed but now I’m working with

public int Money = 500;

GameObject inventory = GameObject.Find("Inventory");
Inventory Inventory = inventory.GetComponent<Inventory>();

public void Marine(){
	Money = Money - 100;
	Debug.Log ("-100 Money");
	Inventory.Marine += 1;
	Debug.Log("+1 Marine.");

	if (Money < 0) {
		Money = Money + 100;
		Debug.Log ("Not Enough Money!");
		Inventory.Marine -= 1;
		Debug.Log ("Marine RESINDED.");
	}
}

public void Assault_Rifle(){
	Money = Money -= 100;
	Debug.Log ("-100 Money");
	Inventory.Assault_Rifle +=1;
	Debug.Log ("+1 Assault Rifle");
	if(Money < 0) {
		Money += 100;
		Debug.Log("Not Enough Money!");
		Inventory.Assault_Rifle -=1;
		Debug.Log ("Assault Rifle RESINDED.");
	}
}

// Update is called once per frame
void Update () {

}

}

line 3-5 being the main problem.