The problem is Trump has changed positions at least 2 times on this and its growing more and more apparent that he can't keep track of what he says nor does he even CARE.
According to The Huffington Post, Trump plans to have Congress appropriate funds to make this happen which is broken promise number one of many on deck I'm sure. Imagine with me, if you will, the conversation the President-Elect would have with Mexico when it becomes time to "reimburse" our country... 
Congratulations America, this is what you elected! Enjoy!