From what I can gather, the way he signed it was basically to say that if they pass it as it currently is, he will withhold any money meant to go to foreign interests. Congress only has the purse strings, they can only open and close the purse, the executive can refuse to issue funds from the purse. Just like how they tried to impeach Trump for withholding money from the Ukraine, when he actually had every right to do so. But they were soo angry, because that's their fraud/bribe money that gets funneled back to them. That's why they used the excuse of a quid pro quo. Because they knew Trump could do it, so they had to feign his motives for doing it were what was the problem.