It's a slippery slope argument. Logicians demand perfect rigour, and label reasoning that fails to attain that a "fallacy". Humans are satisfied with heuristical arguments, which the slippery slope is an example of. Pointing out that it fails to achieve perfect rigour is meaningless; no argument reaches that bar, and demands to pursue it will only result in obfuscation. Instead, try to explain why the heuristic doesn't apply in this case - if you can. The general response to a slippery slope argument is to explain why one step, once achieved, won't lead to any further steps. You will have great difficulty demonstrating that in this case, since government expenditures regularly rise to the level of their budgets (and indeed beyond).

Why would a government not raise expenditures to the level of its budget? Perhaps to pay off debt, but that's only useful in limited circumstances.

On the other hand, certain parts of the political spectrum are very emphatic about wanting to reduce both budget and expenditures, and sometimes manage to do so.