It depends how the Round was applied in the first place.
Is it a Calculation Type? (Manage > Tickets > Calculation Types)
If so, show that configuration, and from that you can create an Update Ticket Calculation action and put that action into the Refund Rule to reverse the Round.
Normal tickets are having Rounding Down to the whole number as in the screenshot above.
I created a Rounding Up to the whole number Calculation Type and created an Action Update Ticket Calculation and attached to Refund Button Rule. Below is my screenshot of the Action
However rounding is not calculated, please let me know if I am missing any config in the Action.
Maybe Amount: should contain some config, since when I enter a figure the calculation is activated.