Learn Arabic (Modern Standard): عندي تسعون دولارا.

In Arabic, for any number from 11 to 99, the item you are counting must be in the singular form. Even though in English we say "ninety dollars" (plural), Arabic grammar requires you to say "ninety dollar".

Why is there an extra alif (ا) at the end of دولاراً?

That extra alif is added to carry the "-an" sound (called tanwin fatha), making the word sound like "dūlāran". The rule for numbers 11 to 99 is that the counted noun must be in the accusative case, which usually means adding this "-an" ending. The alif is just there to act as a "seat" for that sound at the end of the word.

Why is the number written as تسعون (tis'ūn) and not تسعين (tis'īn)?

The "tens" numbers (20, 30, up to 90) change their endings depending on their grammatical role in the sentence, just like regular masculine plural words. The literal structure of this sentence is "Ninety dollars [are] with me." Because "ninety" acts as the subject here, it takes the nominative "-ūn" ending. If it were the object of an action (for example, "I paid ninety dollars"), it would change to تسعين (tis'īn).

I don't see a verb for "to have" here. How does عندي work?

Arabic doesn't use a direct verb for "to have" in the present tense. Instead, it uses prepositions to show possession. عندي (indī) is made of the word عند (at/with) plus the attached pronoun ي (me). So, the sentence literally translates to "With me [are] ninety dollars," which is the standard way to express "I have ninety dollars."