Which is more deserving: paying off a financing loan early, or donating money for an ongoing charity (building a mosque) while continuing to pay the installments?

Building mosques is an ongoing charity (sadaqah jariyah), and its reward is great and diverse. It is one of the best ways to spend money, as the Prophet, peace and blessings be upon him, said: "Whoever builds a mosque, Allah will build for him a similar house in Paradise."

If you can donate to the mosque while also fulfilling your debt when it becomes due, then donating to the mosque is preferable, because it is recommended, and its need might be missed, while it is recommended to hasten in repaying a debt.

However, if your donation leads to delaying the repayment of the debt past its due time, and you cannot find a way to pay it when it becomes due, then you are not permitted to donate; because the more correct view is the prohibition of giving charity from what is needed to repay a debt for which one does not expect to find fulfillment.