There is already a local variable error. Thrown when updating a variable 2 diffrent times in a script

I have coded a script that keeps track of a money balance when buying various things. I have defined the money variable at the beginning of the script with public var money : float = 1000; with 1000 being the starting balance. Updating the variable later in the script with var money = money - fieldCost works perfectly fine for subtracting fieldCost from money, but when using that and, later in the script var money = money - gardenCost to subtract gardenCost from money, compiler throws: NullReferenceException: Object reference not set to an instance of an object on the line of the var money = money - gardenCost. (yes, its a farm game) I might be just noobing this, as I am one, but it is the only thing preventing my game from running and I just can't figure it out!

I haven't yet written the code that sets the boughtField and boughtGarden to true, should that matter.

Code:

 #pragma strict
 
 public var money : float = 1000;
 public internal var boughtField : boolean = false;
 public var fieldCost : float = 150;
 public internal var boughtGarden : boolean = false;
 public var gardenCost : float = 100;
 
 function Start () {
 
 }
 
 function Update () {
     //print balance to console
 //    Debug.Log("CurrentBalance is " + money);
     //run buyplot function
     BuyPlot ();
 }
 
 // processes when player has bought something
 function BuyPlot () {
     //has the player bought a field?
     if (boughtField == true) {
         //do they have enough money to buy a field
         if (money >= fieldCost) {
             //enough money, purchase field
             var money = money - fieldCost;
         }
         //not enough money, sends message
         else {
             Debug.Log("Not enough money to purchase Field!");
         }
     }
     
     //has the player bought a garden?
     if (boughtGarden == true) {
         //do they have enough money
         if (money >= gardenCost) {
             //enough money, purchase garden
             var money = money - gardenCost;
         }
         //not enough money, send message
         else {
             Debug.Log("Not enough Money to buy Garden!");
         }
     }
 }