### 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_
}
            throw JSONRPCError(RPC_WALLET_ERROR, "Can't sign transaction.");
        }
        uint256 txid;
        if (feebumper::CommitTransaction(*pwallet, hash, std::move(mtx), errors, txid) != feebumper::Result::OK) {
            throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
        }
        result.pushKV("txid", txid.GetHex());
    } else {
        PartiallySignedTransaction psbtx(mtx);
        bool complete = false;
        const TransactionError err = pwallet->FillPSBT(psbtx, complete, SIGHASH_DEFAULT, false, true);
        CHECK_NONFATAL(err == TransactionError::OK);
        CHECK_NONFATAL(!complete);
        CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
        ssTx << psbtx;
        result.pushKV("psbt", EncodeBase64(ssTx.str()));
    }
    result.pushKV("origfee", ValueFromAmount(old_fee));
    result.pushKV("fee", ValueFromAmount(new_fee));
    UniValue result_errors(UniValue::VARR);
    for (const bilingual_str& error : errors) {
        result_errors.push_back(error.original);
    }
    result.pushKV("errors", result_errors);
    return result;
},
    };
}
RPCHelpMan bumpfee() { return bumpfee_helper("bumpfee"); }