A woman, single or married, is independent in her takaleef and obligations towards Allah ta'ala, even in financial matters.
A wife is not obliged to financially contribute to household expenditures, however she is able to do so out of her kind generosity and good-will. She is able to count that as her common expenses.
As for long-term loans, and her contributing to getting it paid of, it will not exempt her from khums should she have anything entitled for khums on her khums day.
And Allah knows best.