Allow a customer to pay the sum he want

-- HikaShop version -- : 2.3.3
-- Joomla version -- : 2.5.28
-- PHP version -- : 5.3.10

I need to give the customer the option to make a payment of the sum he want ( maybe a product with undefined price where the user put the price in a text field?).
Is there an existing solution, maybe a plugin?

Regards,

demo.hikashop.com/index.php/en/hikashop/...t-page/donation-page

Thank you very much! It is exactly what I was looking for. It works very well.