It certainly sounds nice in principle, but I don't see how you could carry that out effectively in practice.
Budgeting is a year-by-year process. Every year Congress has to pass a budget resolution (and usually some other supplemental resolutions, continuing resolutions, etc. ... I'm not entirely clear on the details). And short of a Constitutional amendment, one Congress cannot bind the hands of a future Congress.
So it's all well and good to say, "Dear Congressional Agencies, Here are your budget allocations for Fiscal Year 2013. If you spend less than what we've allocated, you'll get to apply it to Fiscal Year 2014. We totally promise. Love, the 112th Congress. XOXOXO." But the 113th Congress can and will do whatever it wants.
And when the difficult negotiations for the FY 2014 start, and everyone has their own priorities, but pretty much all of them involve searching for any actually easy or at least politically easy spending cuts, to be used for other spending, tax cuts, and/or deficit reduction (depending on which Congressperson we're talking about), and the Department of Whatever is insisting that it needs $100 Billion for FY 2014, but everyone can see that it got by just fine on $90 Billion in FY 2013.... the Secretary of Whatever is going to have a difficult enough time persuading Congress to authorize another $100 Billion allocation, let alone getting $110 Billion as a "reward" for not spending the other $10B last year.
