### 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_
CHECK_NONFATAL(min_input_weight == 165);
            if (weight < min_input_weight) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, weight cannot be less than 165 (41 bytes (size of outpoint + sequence + empty scriptSig) * 4 (witness scaling factor)) + 1 (empty witness)");
            }
            if (weight > MAX_STANDARD_TX_WEIGHT) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, weight cannot be greater than the maximum standard tx weight of %d", MAX_STANDARD_TX_WEIGHT));
            }
            coinControl.SetInputWeight(COutPoint(txid, vout), weight);
        }
    }
    if (tx.vout.size() == 0)
        throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
    if (change_position != -1 && (change_position < 0 || (unsigned int)change_position > tx.vout.size()))
        throw JSONRPCError(RPC_INVALID_PARAMETER, "changePosition out of bounds");
    for (unsigned int idx = 0; idx < subtractFeeFromOutputs.size(); idx++) {
        int pos = subtractFeeFromOutputs[idx].getInt<int>();
        if (setSubtractFeeFromOutputs.count(pos))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, duplicated position: %d", pos));
        if (pos < 0)
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, negative position: %d", pos));
        if (pos >= int(tx.vout.size()))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, position too large: %d", pos));
        setSubtractFeeFromOutputs.insert(pos);
    }
    bilingual_str error;
    if (!FundTransaction(wallet, tx, fee_out, change_position, error, lockUnspents, setSubtractFeeFromOutputs, coinControl)) {
        throw JSONRPCError(RPC_WALLET_ERROR, error.original);
    }
}
static void SetOptionsInputWeights(const UniValue& inputs, UniValue& options)