Is it permissible to give the remaining money from the sale of land, which was designated for building a mosque, to an impoverished family that cannot find its daily sustenance?

There is no blame upon you for spending the remaining portion of the land's promised price for the mosque on other charitable causes, especially if the need of those mentioned is greater. Scholars have differed regarding the promise of money: The majority hold that fulfilling it is recommended (mustahabb) but not obligatory (wajib), while the Malikis believe it is binding if it is due to a reason or if the promised person acted upon the promise in some way. The preponderant opinion is that of the majority.