er.
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|false    (boolean) Returns true

Examples:
> bitcoin-cli prioritisetransaction "txid" 0.0 10000
