### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

See inline discussion at https://github.com/bitcoin/bitcoin/pull/26467#discussion_r1269177446

@whitslack:

> Consider the case where I am paying someone on chain under the stipulation that _they_ will eat the mining fee. (Thus, I specify _their_ address in `subtractfeefrom` when I'm constructing my transaction.) Later, they tell me that they can't wait for confirmation any longer and need an urgent fee bump. If I naïvely specify their output as the `reduce_output`, I may end up paying them _**much more**_ than I intended to if I only have large UTxOs in my wallet.


### Expected behaviour

Not sure.

Improving the docs is one option, but this behaviour seem quite counter intuitive.

My own suggestion:

> Should we disable the adding of new inputs when `reduce_output` is set? If someone really intends to do that, they should probably use `outputs`.

### Steps to reproduce

Haven't tried.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

Master after #26467

### Operating system and version

Any

### Machine specifications

_No response_
[&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    const std::shared_ptr<const CWallet> pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_str())) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed. Make sure the tx has at least one input.");
    }
    
    LOCK(pwallet->cs_wallet);
    EnsureWalletIsUnlocked(*pwallet);
    
    std::map<COutPoint, Coin> coins;
    for (const CTxIn& txin : mtx.vin) {
        coins[txin.prevout]; 
    }
    pwallet->chain().findCoins(coins);
    
    ParsePrevouts(request.params[1], nullptr, coins);
    int nHashType = ParseSighashString(request.params[2]);
    
    std::map<int, bilingual_str> input_errors;
    bool complete = pwallet->SignTransaction(mtx, coins, nHashType, input_errors);
    UniValue result(UniValue::VOBJ);
    SignTransactionResultToJSON(mtx, complete, coins, input_errors, result);
    return result;
},
    };
}
static std::vector<RPCArg> OutputsDoc()