Iowa Code § 359.50: Budget amendment.

1. A township budget as finally adopted for the following fiscal year becomes effective July 1. A township budget for the current fiscal year may be amended for any of the following purposes:

a. To permit the expenditure of unexpended, unencumbered cash balances on hand at the end of the preceding fiscal year which had not been anticipated in the budget.

b. To permit the expenditure of amounts anticipated to be available from sources other than property taxation, and which had not been anticipated in the budget.

2. A budget amendment must be prepared and adopted by May 31 of the current fiscal year.