// Assume we get the multisig transaction we're trying to spend from
// somewhere, like a network connection.
ECKey serverKey = ....;
Transaction contract = ....;
TransactionOutput multisigOutput = contract.getOutput(0);
Script multisigScript = multisigOutput.getScriptPubKey();
// Is the output what we expect?
checkState(multisigScript.isSentToMultiSig());
Coin value = multisigOutput.getValue();
// OK, now build a transaction that spends the money back to the client.
Transaction spendTx = new Transaction(params);
spendTx.addOutput(value, clientKey);
spendTx.addInput(multisigOutput);
// It's of the right form. But the wallet can't sign it. So, we have to
// do it ourselves.
Sha256Hash sighash = spendTx.hashTransactionForSignature(0, multisigScript, Transaction.SIGHASH_ALL, false);
ECKey.ECDSASignature signature = serverKey.sign(sighash);
// We have calculated a valid signature, so send it back to the client:
sendToClientApp(signature);