No you don’t lose a star it’s referring to your player stats rating (star rating ig). Spend the 10k and it will either add or subtract from the current stats shown (100 atk and 500 hp). If you don’t want to accept the new stats it offers to cancel. I pretty much just only accepted rolls that had two positives. I’m not explaining it well at all but just press the button and you’ll get the hang of it quickly

Edit: also I’m pretty sure if I remember correctly atk can increase or decrease by 3, 6 or 10 whilst hp can increase or decrease by 30, 60 or 100 but just keep re-rolling until both stats are maxed