### 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_
/
    int nHashType = ParseSighashString(request.params[2]);
    /
    bool sign = request.params[1].isNull() ? true : request.params[1].get_bool();
    bool bip32derivs = request.params[3].isNull() ? true : request.params[3].get_bool();
    bool finalize = request.params[4].isNull() ? true : request.params[4].get_bool();
    bool complete = true;
    if (sign) EnsureWalletIsUnlocked(*pwallet);
    const TransactionError err{wallet.FillPSBT(psbtx, complete, nHashType, sign, bip32derivs, nullptr, finalize)};
    if (err != TransactionError::OK) {
        throw JSONRPCTransactionError(err);
    }
    UniValue result(UniValue::VOBJ);
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << psbtx;
    result.pushKV("psbt", EncodeBase64(ssTx.str()));
    result.pushKV("complete", complete);
    if (complete) {
        CMutableTransaction mtx;
        /
        CHECK_NONFATAL(FinalizeAndExtractPSBT(psbtx, mtx));
        CDataStream ssTx_final(SER_NETWORK, PROTOCOL_VERSION);
        ssTx_final << mtx;
        result.pushKV("hex", HexStr(ssTx_final));
    }
    return result;
},
    };
}
RPCHelpMan walletcreatefundedpsbt()