I've been fighting HMRC recently. They are in a right mess, re covid right now.
My fight is re 2 employees whose NI numbers were mixed up. They had 1 employee earning nothing. Another employee 2 earning his diary, plus employee 2's.
You had letter 1, from HMRC, saying he had 2 jobs?
You've had a 2nd letter? From HMRC saying he owes £3500?
1)when you ring HMRC and speak to the personal tax dept, make sure you note the date, time, name of who you spoke to. Keep detailed notes. It unnerves them.
2)Have you logged online to your husbands account. Take a screenshot of what his tax account says. Does it match his payslips and P60. Do and excel spreadsheet to highlight any differences.
3)Are there duplicates? Does it say he earnt £3000 in April 2020. And then on the next line he earnt £3000 in April 2020 again.
If there are duplicates, requesting old company submit an EYU can be tricky. Especially if they are .... the type to not pay his pension!