Which King or Kingdom first introduced the coins in India for trading (internal or external)?
"The first coins in India were minted around the 6th century BC by the Mahajanapadas of the Indo-Gangetic Plain, and certainly before the invasion of Alexander the Great in the 4th century BC. " Wikipedia
"Earliest Kushan coinage is generally attributed to Vima Kadphises. " Quora
"The first documented coinage is deemed to start with 'Punch Marked' coins issued between the 7th-6th century BC and 1st century AD. These coins are called 'punch-marked' coins because of their manufacturing technique. Mostly made of silver, these bear symbols, each of which was punched on the coin with a separate punch." RBI.org
Quoting from source one (the Hindu):
Thirty copper coins were also found in the excavation area. The structures found on land belonged to the medieval period. "We have also found 30 copper coins. We are cleaning them. After we finish cleaning them, we can give their date," he said.