This is will be rightly challenged in court. He may not like it but Trump does not have the authority to cut off funds that were appropriated through legislation.
The most he can probably do under his executive authority is cut off funds that come from administrative agencies and are dedicated to the enforcement of immigration laws. For example Boston PD system has received a grant from ICE to help pay for temporarily housing illegal immigrants in their jails.
Trump is not the king of Congress’ purse strings. He must know this because he is not a stupid man
If person #1 says ... “I want us to stop killing the innocent.”, and person #2 says ... “The law says we can kill the innocent.”, person #2 may prevail in a court of law, but not in the court of public opinion.