Learn Russian: У меня нет пятидесяти долларов, есть одни рубли. - I do not have fifty dollars, I have only rubles.

While только means 'only' as a general adverb, using the plural form of один (одни) directly before a noun means 'nothing but' that item. In this sentence, одни рубли emphasizes that you have absolutely nothing else in your wallet but rubles.

Why does the number 50 change to пятидесяти here?

The negative phrase у меня нет (I do not have) forces the following words into the genitive case. For compound numbers like 50, both the 'five' part and the 'ten' part decline, turning the nominative пятьдесят into the genitive form пятидесяти.

Since нет requires the genitive case, why is it долларов instead of the genitive singular доллара?

Numbers 5 and above always require the genitive plural for the noun that follows them. Because the word for dollars is tied to the quantity of 50, it takes the genitive plural form долларов. This aligns perfectly with the overarching нет, which also demands the genitive case.

Do I need to repeat у меня before есть одни рубли?

You do not have to. Russian frequently drops repeated words to sound more natural. The у меня from the first half of the sentence carries over perfectly to the second half, making it obvious that you are the one who has the rubles.