There are numerous possibilities here, based on your stated facts. The principal possibility, in my opinion, is that if the original debt collector made an agreement for you to pay the debt, and you paid according to that plan, then you may have satisfied the debt in full, which would make any claim of a subsequent debt collector invalid.
My first move with regard to the new debt collector would be to demand (pursuant to the federal Fair Credit Reporting Act: FCRA) that they verify the debt, and provide: (1) proof of your obligation to pay; and (2) that the provide evidence of the assignment of the debt to their firm. If they can't satisfy both requirements, then I would ignore their demands, because they either don't have authority to collect the debt, or they aren't interested in doing the work to prove their authority -- which means that they probably won't be able to do that with the credit reporting agencies, and that further means that the debt will remain deleted from your credit report.
If they actually provide proof of the debt and their authority to collect, then I would need to review the agreement that you made with the original debt collector re your payment plan, before I could propose next steps. But, that may never become necessary, so you may want to try what I've suggested, and see what happens.
I hope I've answered your question. Please let me know if you require further clarification. And, please provide a positive feedback rating for my answer -- otherwise, I receive nothing for my efforts in your behalf.
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