If I had the money to pay, I could pay.

Literal

Pay money [subject-が] exist-[if-ば], pay-can [concessive-のだが].

仮定法的な表現: 払う金があれば (if I had the money). 払えるのだが uses のだが to express regret — 'I could pay, but (I can't).' The が trails off, implying the condition isn't met.