If I am talking about money. I know that saying Thirty thousand dollars is correct. For eg., 'I gave you Thirty thousand dollars.' But if I am not adding 'dollars' in the end, then would I say Thirty thousands, or Thirty thousand?
You don't use the 's' on numbers in phrases such as 'thirty thousand dollars' because the number here is acting as an adjective. Adjectives in English don't take plurals. Even if you omit the 'dollars' it is understood that 'dollars' whether it is spoken or inferred is the noun.
When you say thousands of dollars you are no longer using 'thousands' as an adjective. It is now a noun. Think of it as a container or package. 2 bags of sugar. 'Bags' is a plural noun right? So too is 'thousands' in thousands of people/dollars/hedgehogs, etc.
Thirty thousand. Numbers do not take a plural form. Thirty thousand volts. Thirty thousand dollars. Thirty thousand tons of coal. You owe me five hundred (dollars/euros/pounds).