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You can't have change when paying a highway toll.
Literal
Fee-charging highway-of passage-fee [object-を] pay-[for-には] [honorific-お]-change [subject-が] exist-[gerund-て]-is-not-allowed-[polite-ません].
お釣りがあってはいけない is a curious construction: '[for there to] be change is not allowed.' The speaker means that the payer must have the exact amount, so no change should be given back. The ~には here is the purpose-plus-topic marker: 'as for paying tolls.' 有料 ('fee-charging, toll, paid') distinguishes from 無料 ('free').