It's even worse than that: it's more like buying unrefundable materials to build a house today on an account/invoice, knowing you will likely be unemployed before the bill comes due, but after the materials are delivered. You're spending money up front that you don't have, and may never have.

If you spend full budget for the first half of the fiscal year, but a final budget gets passed later that cuts your budget in half- you get no remaining money for the year, and then end up firing everyone, instead of half. This is why the federal agencies have the policy of immediately acting on the lowest proposed budget, instead of waiting to see what happens.