Is the amount the wife's father took from her mahr considered a debt upon him, and how can its value be estimated after 30 years, given the change in currency value?

It is not permissible for a woman's guardian to take anything from her dowry except with her consent, with the exception of the father under specific conditions. If your grandfather (your mother's father) took from your mother's dowry in exchange for equipping her (preparing her for marriage), then she has no right to that (the amount taken). However, if he did not equip her, or equipped her from her own private wealth, then the details are as follows: If he took the dowry out of his need and the remainder was sufficient to equip her, then there is no blame on him. But if he took it without need, or if the remaining amount was not sufficient for her needs, then he took what was not lawful for him. If the sheep he bought with the dowry reproduced, then the offspring are a partnership between him and her. If the sheep did not reproduce, then he must return the original money, and it is recommended that he give her more if he is well-off, to mend her feelings.