Organizations make a mountain out of a mole hill when it comes to producing a budget. Sometimes, I get an Excel spreadsheet with 18 tabs, detailing each time they buy paper towels for the office bathroom. Other times, I have to produce a budget for a client....even some with $500,000 budgets.
A Budget, for grant purposes, needs to be a QuickBooks (or similar software) one page budget, listing Revenues and Expenses projected for the current fiscal year. Keep in mind that your organization budget can show some reserves after you deduct expenses from revenue but if you have program budgets (and I encourage you do produce those, too), they should equal out....a zero balance projected at the end of the fiscal year.
Budgets can be amended every time your Board of Directors meets. Remember, it's simply a road map and needs to adjusted as your year progresses.