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She gets paid by the day, not by the month.
Literal
She [topic-は] monthly-salary [not-being-ではなく] daily-wage [object-を] is-receiving.
A clean illustration of two-pole contrast with ~ではなく ('not X but Y'). The conjunctive form of the negative copula sets up the negated alternative; the affirmed answer follows. Common in factual corrections and clear contrasts. The compounds 月給 ('monthly salary') and 日給 ('daily wage') are members of a productive ~給 family — 時給 ('hourly wage'), 年俸 ('annual salary'), 週給 ('weekly pay') — and the choice between them often signals job stability (full-time staff vs. day labor or part-time).