。
If I weren't poor, I'd buy that car.
Literal
Poor [if-not-でなければ] that car [object-を] buying [explanatory-のだが].
でなければ is the negative ば-conditional of the copula — 'if I weren't poor.' のだが at the end combines the explanatory の with だが ('but'), trailing off to imply the regret of an unfulfilled wish. A counterfactual conditional — the speaker IS poor and can't buy the car.