If I owe you money, and somebody else owes me money, yea of course you would sue me, not that other person. But I could write over some of the debt I’m owed to you to clear my debt to you.
And isn’t this exactly how debt enforcement works? You win in court and the court tells the bank (or forces me to tell the bank) to take x amount out of my account and put it into your account. The debt I was owed gets transferred to you, which clears my debt to you.