Why Dharma(धर्म) is pronounced as Dhamma in Buddhism?

First for all I know that the correct **Sanskrit word is धर्म (pronounced as Dharma)** same goes with Nibbana (Nirvana, निर्वाण in Sanskrit) as Siddharth was born in India. So either Pali is just a local dialect as people in Nepal region are not able to pronounce R र. Same goes with native English speaker they roll R, I mean when we say car in India our tongue vibrates, which doesn’t happen with Native english speaker. Why do we use Dhamma instead of Dharma?