Expense.php

//Model: Expense.php
<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Expense extends Model
{
    protected $fillable = [
        'source_id', 'expense_category_id', 'quantity', 'note'
    ];

    //...
    
    public function source()
    {
      return $this->belongsTo(Source::class);
    }
}

ExpenseController.php

//Controller: ExpenseController.php
<?php

namespace App\Http\Controllers;

use App\Expense;
use App\Http\Requests\ExpenseRequest;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Auth;
use Illuminate\Support\Facades\DB;

class ExpenseController extends Controller
{
    //...

    public function store(ExpenseRequest $request)
    {
        DB::beginTransaction();

        try {
            $expense = new Expense($request->all());
            Auth::user()->expenses()->save($expense);

            $expense->source->subExpense($expense);
            DB::commit();
        } catch (\Exception $e) {
            DB::rollback();
            return response()->json(['message' => $e->getMessage()], 500);
        }

        return response()->json($expense, 201);
    }
}

Source.php

//Model: Source.php

<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Source extends Model
{
    protected $fillable = [
        'name', 'quantity'
    ];

    //...

    public function expenses()
    {
      return $this->hasMany(Expense::class);
    }

    public function subExpense($expense) {
        $this->quantity = $this->quantity - $expense->quantity;
        $this->save();
    }
}

Algo así debería quedar pero en expense
//Expense model

public function subExpense($expense) {
$this>quantity-=$this->source()->quantity// puedes acceder al source por medio del belongsTo ya que es
parte del mismo modelo
}