The dumpprivkey RPC returns the wallet-import-format (WIF) private key corresponding to an address. (But does not remove it from the wallet.)
dumpprivkey "address"
*bitcoin-cli help dumpprivkey
dumpprivkey "address" Reveals the private key corresponding to 'address'. Then the importprivkey can be used with this output Arguments: 1. "address" (string, required) The bitcoin address for the private key Result: "key" (string) The private key Examples: > bitcoin-cli dumpprivkey "myaddress" > bitcoin-cli importprivkey "mykey" > curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "dumpprivkey", "params": ["myaddress"] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/