Is the expiatory payment (kaffara) for the days I missed fasting, which span several Ramadans, to be calculated based on the amount previously paid to my husband (three hundred pounds) or on the current amount? And is it permissible to give this money to a mosque that hosts iftar meals for fasting individuals (Ma'idat al-Rahman)?

The questioner's obligation to make up for the missed fasts is not discharged except by fasting all the days she missed. If she does not know their number, she should fast what she believes is most likely sufficient for her. As for expiation (kaffarah), she must feed one poor person for each day she delayed making up the fast without a valid excuse. The amount is one mudd of food. It is not permissible to pay its value in money except out of necessity, and that should be at its current value. It is permissible to give the expiation to those in charge of providing Iftar for fasting people, provided that the beneficiaries are poor and needy. As for the amount the husband must return, it is the 300 that he took from her money.