A web based interface to the Bitcoin API JSON-RPC

Command: prioritisetransaction

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.)

Parameter #1—the TXID of the transaction to modify
Parameter #2—the change to make to the virtual priority
Parameter #3—the change to make to the virtual fee
Result—true if the priority is changed

The TXID of the transaction whose virtual priority or fee you want to modify, encoded as hex in RPC byte order.
If positive, the priority to add to the transaction in addition to its computed priority; if negative, the priority to subtract from the transaction’s computed priory. Computed priority is the age of each input in days since it was added to the block chain as an output (coinage) times the value of the input in satoshis (value) divided by the size of the serialized transaction (size), which is coinage * value / size.
Warning: this value is in satoshis, not bitcoins. If positive, the virtual fee to add to the actual fee paid by the transaction; if negative, the virtual fee to subtract from the actual fee paid by the transaction. No change is made to the actual fee paid by the transaction.
prioritisetransaction <txid> <priority delta> <fee delta>
Accepts the transaction into mined blocks at a higher (or lower) priority

1. "txid"       (string, required) The transaction id.
2. priority_delta (numeric, required) The priority to add or subtract.
                  The transaction selection algorithm considers the tx as it would have a higher priority.
                  (priority of a transaction is calculated: coinage * value_in_satoshis / txsize) 
3. fee_delta      (numeric, required) The fee value (in satoshis) to add (or subtract, if negative).
                  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.

true              (boolean) Returns true

> 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;'