If you were scammed 100m by someone, and you or someone else are given a chance to retrieve said 100m from said scammer, you are allowed to do this. You are not allowed to take more than what has been scammed from you or others. Generally it's a good idea to notify a staff member that you plan on counterscamming someone but on a thread a few months back, a staff member stated that you don't technically need to. Even if you weren't scammed by someone, if you have the opportunity to retrieve money from a scammer who has outstanding debts either here or offsite, I believe you are allowed to hold their money, however i'm fairly sure this isn't suggested and is usually best done mainly by staff members. It's best to stay out of any sort of counterscamming/attempts unless it involves you directly, and like I said better to notify a staff member than not.