Given that the Trump administration has asserted the right to rescind or impound congressionally appropriated funds at will, why should Democrats agree to any deal to reopen government?

The Trump administration, and the Republican majority in Congress, have made clear that Congressionally allocated spending is more of a suggestion rather than a binding obligation. This means that any deal the Democrats agree to might go up in smoke as soon as the president decides to rescind or impound funding. Why should the Democrats trust that any funding they approve will actually go where they vote for it to go?