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It could very well make him not eligible to stay on the loan with a new open active collection. I would be on the phone with my mortgage lender first thing in the morning to find out my next move credit wise. It may be something that if he can show insurance paid the bill or payment wasn't sent from the insurance company, possibly can be removed, then a rapid rescore done. This thread may get moved also.
Chase may or may not re-run your husbands credit. Prudent underwriting practices would be that credit is re-checked, but it's up to each lender to determine if they would or not. They will see the new collection though, as prior to closing a "credit refresh" is done (it's not a hard inquiry, but just updates the accounts on your report). At that time the underwriter would determine if a new credit check (hard inquiry) would need to be done. As has been mentioned, inform your loan officer ASAP of this so you aren't waiting to just before closing to address it. A 675 score still should qualify for most programs, but will likely result in higher rates/payments. The collection probably won't have to be paid (since it's medical), but Chase is more conservative than most lenders so just be aware that they may require. Hope it works out for you.
what shane said....
would be a good question for your loan officer