I imagine that “the art of the deal” is him explaining what he got for selling his soul to the devil, not that he wrote it himself.

Donald: I want to renegotiate our deal

Devil: I already got your soul

Donald: I can give you the soul of my wife, children and all relatives.

Devil: In return for what? Wisdom? Fame? Fortune?

Donald: I already got those, I want to know everything about decorating, fashion and art.

Devil: Deal!