This doesn?t seem to add up
$450,000 / 2 = $225,000, i.e. each half is worth $225,000
Half the existing loan is $120,000. This is how much your spouse would pay out to the bank to buy your half (or you hers/his).
So the new deductible loan would be $120,000 + $225,000 (100% of the half) =...