Lenders policies vary on this. Some allow it, so your only hope is to ask your particular lender given your particular circumstances, will they allow you to take over the mortgage and retain the tracker. I have seen cases where it's been no problem. I have seen cases where they just refused to take the second borrower off the mortgage. And I have seen cases where they agreed to take him off the mortgage as long as she reduces the negative equity by €50k.
If your bank refuses, your husband can sign a paper surrendering all rights to the property. Unfortunately, he remains on the mortgage and this will hinder him from getting another mortgage.
So
1) Ask you bank. If they agree, off you go.
2) If they refuse, talk to your husband about the side-agreement.
3) If he agrees, get your solicitor to draft it up.
Brendan