Which of these sentences is correct?
Rupees are interchangeable with dollars
or
Rupees is interchangeable with dollars.
To keep it simple... Nouns have 2 groups for number: count and non-count. If you can count them individually, they are count. If you can only measure them, they are non-count (or uncountable). They are handled differently for S-V agreement. Rupees are countable, and take the form of the verb applying to plural. Money, on the other hand, is a concept, uncountable, but still measurable. It takes the singular form of the verb. Thus we would have:
Money is...
but,
Rupees are.
Other example of uncountable nouns: sugar, trouble, gasoline
Examples of count: apple, problem, gas-tank
There some exceptions to be careful of such as school subjects, diseases, names of countries ending in s, etc. and a few others like "news" .
examples: Mathematics, Physics, Statistics, Measles, Mumps,
all of these, including "news" is always singular because the words end in "s". They are not plural.
Rupees are interchangeable with dollars is the correct usage