"Trump is the one who is refusing to negotiate."
The thing is, there's nothing to negotiate in terms of The Wall. It's yes or no. The Dems can't part fund it and Trump can't accept less than the full amount based on their current positions. One side or the other has to back down and either forget it, or get something else in exchange.
Having said that, Trumps campaign promise was that US tax payers would not have to pay for The Wall, so Trump is the one breaking the promise and putting new and unexpected demands on the tax payer. The Dems are just trying to assist him in keeping is promise of no taxation for The Wall.