Obviously a corrupt belief is much worse than a sin. Therefore, a heretic whose corrupt creed has not reached the extent of unbelief (Kufr) cannot be Kufw for a Sunnī woman. If a heretic’s corrupt creed has reached unbelief (i.e. he is a Murtad) then marriage with him is invalid because he is not even a Muslim, let alone be Kufw.
(Baĥār-e-Sharī’at, part 7, pp. 54)
Question: What does Kufw in wealth mean?
Answer: Kufw in wealth means that the man should be capable of paying Maĥr-e-Mu’ajjal and providing food, clothing etc. If he does not work, he must have one month’s expenses, otherwise his daily earnings must be enough to provide for the woman’s daily needs. He is not required to be as wealthy as her.
(Baĥār-e-Sharī’at, part 7, pp. 54)
Question: Is Kufw important in a marriage between a non-pubescent boy and girl?
Answer: Non-pubescent boys and girls are not capable of making a marriage proposal nor can they give consent. Instead, their guardian (Walī) will perform these tasks on behalf of them. A non-pubescent cannot marry without a guardian. However, even in this issue in some cases it is a precondition for Nikah that the boy meet the standard of Kufw. For example, if a non-pubescent girl is married to a boy by a distant guardian (Walī Ab’ad) in the absence of her father or grandfather, then in this case it is necessary for the boy to be Kufw for her. Similarly, a non-pubescent girl’s father can only marry her to a non-Kufw boy once. After this one marriage, the father cannot marry any of his other daughters to a non- Kufw boy.