Yes, you can perform refunds on the spot for the full or partial amount (subject to the daily refund limit of $1,000).
Your customer and the card they used to make the original payment will need to be in front of you, as they will need to enter their card's security PIN. Refunds can only be made to the same card and account that the payment was made with originally.
Refunds are credited the next business day if processed before 10pm Monday to Friday.