### 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_
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    if (!request.params[0].isNull() && !request.params[0].get_str().empty()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Dummy value must be set to \"\"");
    }
    UniValue sendTo = request.params[1].get_obj();
    mapValue_t mapValue;
    if (!request.params[3].isNull() && !request.params[3].get_str().empty())
        mapValue["comment"] = request.params[3].get_str();
    UniValue subtractFeeFromAmount(UniValue::VARR);
    if (!request.params[4].isNull())
        subtractFeeFromAmount = request.params[4].get_array();
    CCoinControl coin_control;
    if (!request.params[5].isNull()) {
        coin_control.m_signal_bip125_rbf = request.params[5].get_bool();
    }
    SetFeeEstimateMode(*pwallet, coin_control, request.params[6], request.params[7], request.params[8], false);
    std::vector<CRecipient> recipients;
    ParseRecipients(sendTo, subtractFeeFromAmount, recipients);
    const bool verbose{request.params[9].isNull() ? false : request.params[9].get_bool()};
    return SendMoney(*pwallet, coin_control, recipients, std::move(mapValue), verbose);
},
    };
}
RPCHelpMan settxfee()