If you reroll a shop and none of the new pieces are units already on your board or bench, you're refunded 1–2 gold, making that reroll either free or half price (if an owned piece does show up, you get nothing). The same check also runs on the free shop at the start of each round.
The amount scales with consecutive whiffs: the first four misses in a row always refund 1 gold, then 2 gold becomes possible from the 5th miss onward (reaching a coin flip by the 8th). Any shop that shows an owned piece resets the streak.
Very useful for roll-heavy boards, especially early when you own few units, so most rerolls whiff and trigger the refund.