He never said he’d spend a billion of his own money. He said he’d self fund the primary. He has won the primary so it’s time for everyone else to chip in and support him in the General.
Actually, Trump DID say he’d spend a billion of his own money. He also never specified that he only intended to self-fund his own primary.
http://www.cbsnews.com/news/donald-trump-willing-to-spend-1-billion-on-presidential-bid/