2 Chainz has a lotta nerve skipping out on his Rolls-Royce tab ... according to a high-end auto broker who's suing to get the dough.

Highline Holdings Group says 2 Chainz hired them to lock down a Rolls-Royce Phantom Drophead, but when it came time to pay, the rapper was a little absent-minded. According to the docs, obtained by TMZ, Highline negotiated a $262,212 price, to which Chainz agreed and paid most of that sum.

He had an outstanding balance of $18,317, and when Highline contacted him he refused to fork over one more penny ... according to the suit. Even though the exotic ride was delivered to his home in Georgia, the broker insists 2 Chainz won't pay.

Highline's suing to get the balance plus interest. So far, no word from 2 Chainz.