Trump is free to spend less than budgeted.

Budgets are an approval to spend up to a certain amount, not a requirement to spend a certain amount.

We passed laws about that because of a previous Republican president who was also fond of executive overreach.

Exactly how else are you supposed to get this tested by the courts?

Not without approval from Congress:

"The Impoundment Control Act of 1974 provides that the president may propose rescission of specific funds, but that rescission must be approved by both the House of Representatives and Senate within 45 days. In effect, the requirement removed the impoundment power, since Congress is not required to vote on the rescission and, in fact, has ignored the vast majority of presidential requests."

Source: https://en.wikipedia.org/wiki/Impoundment_of_appropriated_fu...