Case:


Ruling: Def is considered to have stolen the money from her daughters' account. In general, a thief pays only the value of the object at the time of the theft. We do not factor in what the profits the object could have been had it not been taken, as that is gerama (indirect damage).
However, as the fund's overseer, def was a shomer (watchman). The Chatam Sofer (CM 140) says that since a shomer's payments are usually for damage caused indirectly, she would be responsible even for gerama. However, he referred only to an object's value at the time of damage, which would not indicate that def has to pay for potential earnings after withdrawal of funds. The Rivash (396) says, regarding a case where one watching wheat illegally sold it when its price was low but rebounded later, that he only has to pay according to the price when he stole them, not for future appreciation.
What about payment as one who damages? The gemara (Bava Metzia 99b) says that if one broke another's barrel that is worth four zuz on a regular day and five on a market day, he either returns a barrel on a market day or five zuz on another day. Rashi says that even if he damaged it on a day when it was worth four, he pays five because it is sometimes that expensive. The K'tzot Hachoshen (291:1) explains that since the price will definitely go up, the damager caused loss of the extra value. The Netivot Hamishpat (291:2) argues against the notion of semi-direct damage payment. Rather the payment was because he was a worker, who has a special obligation when performing work in a manner that causes damage. In our case, since the bank's interest payments were set, the loss is definite. Def is also entrusted to secure the investment. Thus both the K'tzot and Netivot would obligate lost future interest.
The Yerushalmi (Bava Metzia 5:3) says that one who didn't invest funds given to him for that purpose need not pay. The Raavyah explains that this is because the gains were not certain. The Nimukei Yosef (Bava Metzia 61b) says that it is because the "broker" did not explicitly accept responsibility. In our case, though, the interest was set and the overseer accepted responsibility to ensure it would be received. However, according to the Shach (CM 292:15) one who does not invest is still an indirect damager and is exempt. The Chatam Sofer (CM 178) says that a compromise should be made between the sides. Beit din thus obligates def to pay the amount the investment would have been worth had it been due on the day def withdrew the money.