Laravel 5.8: Parameter passed to the event listener is getting null

I have created an Event called UserWalletNewTransaction.php and added this to it:

public $transaction;

public function __construct($transaction) {
    $this->$transaction = $transaction;
}

And also registered it at EventServiceProivder.php:

use App\Listeners\UserWalletNotification;

protected $listen = [
    UserWalletNewTransaction::class => [
        UserWalletNotification::class,
    ],

Now in order to fire this event at the Controller, I coded this:

$newTransaction = UserWalletTransaction::create(['user_id' => $user_id, 'wallet_id' => $wallet_id, 'creator_id' => $creator_id, 'amount' => $amount_add_value, 'description' => $trans_desc]);

event(new UserWalletNewTransaction($newTransaction));

Then at the Listener, UserWalletNotification.php, I tried:

public function handle(UserWalletNewTransaction $event) {
    dd($event->transaction);
}

But it returns null somehow.

However if I dd($event) instead, I get this:

So what is going wrong here? I can properly insert new record at user_wallet_transactions and the variable $newTransaction should contain the transaction information but it's not.

Here is also the Model UserWalletTransaction.php:

protected $fillable = ['user_id','wallet_id','amount','description','creator_id'];
protected $table = 'user_wallet_transactions';

I would really appreciate any idea or suggestion from you guys about this...

Thanks in advance.

---

UPDATE #1:

If I do dd($newTransaction) at the Controller before event(), I get this:

---

UPDATE #2:

Here is the full code of event, UserWalletNewTransaction.php:

class UserWalletNewTransaction
{
    use Dispatchable, InteractsWithSockets, SerializesModels;

    /**
     * Create a new event instance.
     *
     * @return void
     */
    public $transaction;

    public function __construct($transaction)
    {
        $this->$transaction = $transaction;
    }
}

---

UPDATE #3:

Here is the full result of dd($event) at the Listener:

App\Events\UserWalletNewTransaction {#2533 ▼
  +newTransaction: null
  +socket: null
  +"{"user_id":"373","wallet_id":"2","creator_id":2,"amount":"-60","description":null,"updated_at":"2021-07-18 13:33:59","created_at":"2021-07-18 13:33:59","id":61}": App\UserWalletTransaction {#2535 ▼
    #fillable: array:5 [▼
      0 => "user_id"
      1 => "wallet_id"
      2 => "amount"
      3 => "description"
      4 => "creator_id"
    ]
    #table: "user_wallet_transactions"
    #connection: "mysql"
    #primaryKey: "id"
    #keyType: "int"
    +incrementing: true
    #with: []
    #withCount: []
    #perPage: 15
    +exists: true
    +wasRecentlyCreated: true
    #attributes: array:8 [▼
      "user_id" => "373"
      "wallet_id" => "2"
      "creator_id" => 2
      "amount" => "-60"
      "description" => null
      "updated_at" => "2021-07-18 13:33:59"
      "created_at" => "2021-07-18 13:33:59"
      "id" => 61
    ]
    #original: array:8 [▼
      "user_id" => "373"
      "wallet_id" => "2"
      "creator_id" => 2
      "amount" => "-60"
      "description" => null
      "updated_at" => "2021-07-18 13:33:59"
      "created_at" => "2021-07-18 13:33:59"
      "id" => 61
    ]
    #changes: []
    #casts: []
    #dates: []
    #dateFormat: null
    #appends: []
    #dispatchesEvents: []
    #observables: []
    #relations: []
    #touches: []
    +timestamps: true
    #hidden: []
    #visible: []
    #guarded: array:1 [▼
      0 => "*"
    ]
  }
}

I just figured out the problem... You are doing $this->$transaction when it should be $this->transaction.