What the hell are you saying? Why would Trump hesitate to sign the law, but then have to worry somehow about withholding funds? He had just observed Obama do it seven times.
If Trump opposed the foreign policy inherent in the Defense Appropriations Act, which explicitly specified delivering aid to Ukraine, he should have refused to sign it.
Once he signs the bill into law he has obligations to faithfully execute it.
If he changes his mind he can ask Congress to change the funding, but he can't just ignore what the law says.