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If you don't execute an "agreed judgment", the creditor will notice the matter for a hearing and ask the court to enter a judgment against you. If you go this route, it is likely that the creditor will assess the court costs and attorney's fees to you, thereby making the balance you owe go up even more.
Thus, if you owe the money and have no defenses to the claim, you should probably just sign the agreed judgment and avoid unnecessary costs.