I signed a Judgement prematurely

Hello I signed a judgement because I was told by the collector that the court date was the following week and to avoid this, I will have to sign the judgement and send it back before the court date. I told the collector that I could only afford $25.00 per month, so we agreed on $25.00 per month for 11 months and 150.00 for the next 6 years. I told him I could not afford the $150. He said he just needed to put something down for the courts and that I could call within the 11 months to continue the $25.00 per month payments. Fast forward 1.5 years later, the debt collection agency will now only accept $150.00 per month. I wish I had the time to think, but all of this came out of nowhere. I wasn’t aware of this company. The first time I knew about this debt collector was through the papers that was served to me. In addition, I found out later that a court date had not been set and I have proof that the amount they said I owe in the judgement is incorrect. Is it wise to write the debt collector about these discrepancies? Can I get the judgement set aside?