For the purposes of this article, the unencumbered funds of the district are the sum of all money, uncollected taxes, and other uncollected accounts belonging to or due to such district, in excess of an amount sufficient to pay all claims and accounts against the district, including both claims and accounts lawfully payable from the funds of such district on the date of annexation or incorporation, and claims and accounts which will become payable from such funds by reason of lawful contracts in force on that date.