Trump loaned the money to his campaign, he could retire his campaign costs with donations at a later point. And he’s taken in a couple million in donations from the public beyond the money he himself fronted.
Plenty of articles about it, choose you own source of liking.
This was the first match on google
Is Donald Trump self-funding his campaign? Sort of
By Lauren Carroll on Wednesday, February 10th, 2016 at 4:00 p.m.
...”Trump’s campaign brought in about $19.4 million by the end of 2015. Trump contributed nearly $13 million of that himself. Most of the remainder comes from individual contributions, which federal law caps at $2,700 per candidate per election.”...
“There is, though, a ‘donate’ button on Trump’s website.”
and
...”But in the last quarter, Trump gave his campaign a $10.8 million loan, turning that balance around.”
“That brings us to the second caveat: The vast majority of Trump’s contributions to his own campaign — about $12.6 million — are loans rather than donations. This means he could expect to eventually recoup these funds.”
“Further, of the approximately $12 million Trump’s campaign spent in 2015, about $2.7 million went toward reimbursing Trump-affiliated companies for services provided to the campaign, such as traveling in his own plane and helicopter, according to a New York Times analysis.”