고마워요. Peter. It's all clear now.

You already said what 불 is. It means American dollars (USD). "불" also has secondary meanings of "fire" or "light", but obviously these aren't related to money. Using 달러 as a counter is easy. It's almost identical to English. 20 달러 (이십 달러) = 20 dollars. 불 is equivalent to 달러 in the sense that both can be used as counters. 20 달러 = 20 불 = 20 dollars. You should use Sino-Korean numbers (일, 이, 삼, 사..) with both counters! You do NOT change the 육 in 육년 to 유. However, the word is pronounced "융년" in actual conversation. I bet that's the reason why you asked that question. Remember that pronunciation. It's important.