Hello,
The Paypal "classical" plugin do not use any specific API or "Paypal Application" ; which mean that the plugin cannot ask for a refund to Paypal, there is no API for that.
If you want to do so, it will require the development of a new Paypal plugin to support one other Paypal service and have access to an API to control the transactions.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.