The clause you're referring to is the one granting Congress the power to collect taxes, and then limiting how that tax money can be spent. You can tell that it's a limitation rather than a grant because "provide for the common defense" would otherwise make the subsequent clauses redundant:

> To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;

> To provide and maintain a Navy;

Which wouldn't make much sense given that they're in the same article and were passed at the same time.

But let's suppose we're talking about a law where Congress is allocating tax money to something within their enumerated powers. Then that money goes to the executive. It would be pretty hard to argue that the executive could then spend the money on something else, but what if they don't spend it at all? Congress gave them money with the condition that they do a particular thing with it, and they don't want the money? It's not obviously true that they're required to take it, and would be consistent with the notion of checks and balances that they could turn it down.