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Lawyers get a huge payout when they win a case.
Literal
Lawyer [topic-は] trial [in-で] wins [when-と] huge-sum [gen-の] reward [obj-を] can-receive.
~と as 'when/if' for a general truth: the condition leads to the consequence as a natural rule. もらえる is the potential of もらう ('receive'), so 'can receive' — here functioning like 'gets to receive.' 巨額 ('huge sum') is a formal Sino-Japanese compound often paired with money-related nouns: 巨額の報酬, 巨額の損失.