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Are they requiring that it be paid prior to any loan approval, or only that you have at least entered into a repayment agreement/plan?
Make the credtior a settlement offer for whatever amount you can afford,and hope they accept.
The mortgage underwriter is concerned with the presence of the debt per se.
That applies,now that they are aware of the asserted debt, regardless of whether it is reported or removed from your credit report.
An FCRA dispute thus wont address the issue of legitimacy of the debt.
Additionally, the debt is apparently not asserted to be inaccurate.
I agree that pursuing an FCRA dispute at this point would likely be counter-productive.