diff --git a/app/Rules/QuantityValidationRule.php b/app/Rules/QuantityValidationRule.php
new file mode 100644
index 0000000..ffee127
--- /dev/null
+++ b/app/Rules/QuantityValidationRule.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace App\Rules;
+
+use App\Models\Portfolio;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+class QuantityValidationRule implements ValidationRule
+{
+    /**
+     * Create a new rule instance.
+     *
+     * @return void
+     */
+    public function __construct(
+        protected Portfolio $portfolio, 
+        protected string $symbol, 
+        protected string $transactionType
+    ) {
+        $this->portfolio = $portfolio;
+        $this->symbol = $symbol;
+        $this->transactionType = $transactionType;
+    }
+
+    /**
+     * Validate the attribute.
+     *
+     * @param  string  $attribute
+     * @param  mixed  $value
+     * @param  \Closure  $fail
+     * @return void
+     */
+    public function validate(string $attribute, mixed $value, \Closure $fail): void
+    {
+        if ($this->transactionType == 'SELL') {
+            $holding = $this->portfolio->holdings()->symbol($this->symbol)->first();
+            $maxQuantity = $holding ? $holding->quantity : 0;
+            
+            if ($value > $maxQuantity) {
+                $fail(__('The quantity must not be greater than the available quantity.'));
+            }
+        }
+    }
+}
diff --git a/lang/en.json b/lang/en.json
index f462886..2ea0f49 100644
--- a/lang/en.json
+++ b/lang/en.json
@@ -134,6 +134,7 @@
     "Symbol": "Symbol",
     "Quantity": "Quantity",
     "Quantity Owned": "Quantity Owned",
+    "The quantity must not be greater than the available quantity.": "The quantity must not be greater than the available quantity.",
     "Average Cost Basis": "Average Cost Basis",
     "Market Value": "Market Value",
     "52 week": "52 week",
diff --git a/lang/es.json b/lang/es.json
index 60efdbe..a11c27d 100644
--- a/lang/es.json
+++ b/lang/es.json
@@ -134,6 +134,7 @@
     "Symbol": "Símbolo",
     "Quantity": "Cantidad",
     "Quantity Owned": "Cantidad de propiedad",
+    "The quantity must not be greater than the available quantity.": "La cantidad no debe ser mayor que la cantidad disponible.",
     "Average Cost Basis": "Costo Promedio",
     "Market Value": "Valor de Mercado",
     "52 week": "52 semanas",
diff --git a/resources/views/livewire/manage-transaction-form.blade.php b/resources/views/livewire/manage-transaction-form.blade.php
index f160304..8493ee3 100644
--- a/resources/views/livewire/manage-transaction-form.blade.php
+++ b/resources/views/livewire/manage-transaction-form.blade.php
@@ -3,10 +3,12 @@
 use App\Models\Transaction;
 use App\Models\Portfolio;
 use App\Rules\SymbolValidationRule;
+use App\Rules\QuantityValidationRule;
 use Illuminate\Support\Collection;
 use Livewire\Attributes\{Computed};
 use Livewire\Volt\Component;
 use Mary\Traits\Toast;
+use Illuminate\Validation\Rule;
 
 new class extends Component {
     use Toast;
@@ -34,7 +36,12 @@ new class extends Component {
             'transaction_type' => 'required|string|in:BUY,SELL',
             'portfolio_id' => 'required|exists:portfolios,id',
             'date' => 'required|date_format:Y-m-d',
-            'quantity' => 'required|min:0|numeric',
+            'quantity' => [
+                'required', 
+                'numeric', 
+                'min:0', 
+                new QuantityValidationRule($this->portfolio, $this->symbol, $this->transaction_type)
+            ],
             'cost_basis' => 'exclude_if:transaction_type,SELL|min:0|numeric',
             'sale_price' => 'exclude_if:transaction_type,BUY|min:0|numeric',
         ];