Even if the girls are happy to do the above, it’s still not so simple, as the halachah is (554:20) that one shouldn’t greet his friend on Tisha B’Av. The Magen Avraham (s.k. 21) writes that included in the above, is that one shouldn’t send a present to a friend on Tisha B’Av either, as sending presents is a way of greeting people and is included in the issur of greeting a friend on Tisha B’Av. The Mishnah Berurah (s.k. 41) rules accordingly. Based on this it would seem that giving the money as a present is also not such an ideal solution.
Some suggest, that what the parents should do is, on Tisha B’Av they should guarantee the girls that they will pay, however, they should only give the money after Tisha B’Av. However, this solution is also not so simple, as the Shu”t Hisorarus Teshuvah (3:355) says, that guaranteeing a present is also a problem, as it’s equivalent to greeting someone on Tisha B’Av.
