### 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_
if (new_coin_control.IsExternalSelected(txin.prevout)) {
            
            int64_t input_weight = GetTransactionInputWeight(txin);
            
            
            
            
            SignatureWeights weights;
            TransactionSignatureChecker tx_checker(wtx.tx.get(), i, coin.out.nValue, txdata, MissingDataBehavior::FAIL);
            SignatureWeightChecker size_checker(weights, tx_checker);
            VerifyScript(txin.scriptSig, coin.out.scriptPubKey, &txin.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, size_checker);
            
            input_weight += weights.GetWeightDiffToMax();
            new_coin_control.SetInputWeight(txin.prevout, input_weight);
        }
    }
    Result result = PreconditionChecks(wallet, wtx, require_mine, errors);
    if (result != Result::OK) {
        return result;
    }
    
    CAmount output_value = 0;
    for (const auto& old_output : wtx.tx->vout) {
        output_value += old_output.nValue;
    }
    old_fee = input_value - output_value;
    
    
    
    std::vector<CRecipient> recipients;