Is it permissible for sisters to donate charity to the mosque in the form of food and other items, and then buy them back again with the intention of helping the mosque with double the amount, knowing the hadith of Umar ibn al-Khattab, may Allah be pleased with him, in which the Messenger of Allah, peace and blessings be upon him, forbade him from buying back his charity?

It is impermissible for a donor to buy back their own charity, based on what Al-Bukhari and Muslim narrated from Umar (may Allah be pleased with him) that he donated a horse in the way of Allah, then wished to buy it back. The Prophet (peace be upon him) forbade him from doing so, saying: "Do not buy it back, even if you were given it for a single dirham, for the one who takes back his charity is like a dog that returns to its vomit." This is the view of Imam Ahmad and it is the most preponderant opinion, because a donor buying back their charity is a means of retrieving something from it, which spoils the charity. However, if the charity returns to the donor through inheritance, a gift, a bequest, or if they take it back from their debt, or if the Imam returns it to them after taking possession of it because they are among its rightful recipients, then it is permissible for them to take it.

Therefore, it is not permissible for you to buy back what you have donated to the mosque. However, it is permissible for one of you to buy back what the other has donated.