prioritisetransaction <txid> <dummy value> <fee delta>

The prioritisetransaction RPC adds virtual priority or fee to a transaction, allowing it to be accepted into blocks mined by this node (or miners which use this node) with a lower priority or fee. (It can also remove virtual priority or fee, requiring the transaction have a higher priority or fee to be accepted into a locally-mined block.)

*bitcoin-cli help prioritisetransaction

prioritisetransaction   
Accepts the transaction into mined blocks at a higher (or lower) priority

Arguments:
1. "txid"       (string, required) The transaction id.
2. dummy          (numeric, optional) API-Compatibility for previous API. Must be zero or null.
                  DEPRECATED. For forward compatibility use named arguments and omit this parameter.
3. fee_delta      (numeric, required) The fee value (in satoshis) to add (or subtract, if negative).
                  Note, that this value is not a fee rate. It is a value to modify absolute fee of the TX.
                  The fee is not actually paid, only the algorithm for selecting transactions into a block
                  considers the transaction as it would have paid a higher (or lower) fee.

Result:
true              (boolean) Returns true

Examples:
> bitcoin-cli prioritisetransaction "txid" 0.0 10000
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "prioritisetransaction", "params": ["txid", 0.0, 10000] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/