Hi,
I don't have the code of the plugin so I can't say why it doesn't work properly for your case.
However, here is what I can say:
The payment plugins are supposed to always give the HikaShop default return URL to the payment gateway. Then, if they want HikaShop to redirect somewhere else, they can have their return URL in the payment_params of their plugin named "return_url". That way, on the default return URL of HikaShop, after clearing the cart, it will take that URL in that parameter and redirect to that return URL you specify in the settings of the payment plugin.
So if your developer had followed the instructions we provide in the example payment plugin of our developer page, the situation you're describing shouldn't happen, unless you have an old version of HikaShop with a bug on the return URL of HikaShop which doesn't clear the cart properly. However, with the latest version of HikaShop, this shouldn't happen.