create a custom validation in laravel where input value must be greater than the existing value in the database

I had the same issue solved like this, though someone already gave the solution in the comments section.

/**
     * @param  array  $data
     * validates and Stores the application data
     *
     */
    public function sendMoney(Request $request)
    {
      //get the value to be validated against
        $balance = Auth::user()->balance;
        $validator = Validator::make($request->all(), [
            'send_to_address' => 'required',
            'amount_to_send' => 'required|max:'.$balance.'|min:0.01|numeric',
        ]);
     //some logic goes here
    }

Depending on your use case you could modify...

Happy Coding

Assuming you have Printer model which contains the page_count column.
You can define a custom validation rule in your AppServiceProvider's boot() method.

public function boot()
    {
        //your other code

        Validator::extend('page_count', function($attribute, $value, $parameters, $validator) {
                    $page_count = Printer::find(1)->first()->value('page_count'); //replace this with your method of getting page count.
                    //If it depends on any extra parameter you can pass it as a parameter in the validation rule and extract it here using $parameter variable.
                    return $value >= $page_count;
                });

        //your other code
    }

Then, you can use it in your validation rule like below

'page_count' => 'required|page_count'

Reference: Laravel Custom Validation