### 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 (!res) {
        errors.push_back(Untranslated("Unable to create transaction.") + Untranslated(" ") + util::ErrorString(res));
        return Result::WALLET_ERROR;
    }
    const auto& txr = *res;
    
    new_fee = txr.fee;
    
    mtx = CMutableTransaction(*txr.tx);
    return Result::OK;
}
bool SignTransaction(CWallet& wallet, CMutableTransaction& mtx) {
    LOCK(wallet.cs_wallet);
    if (wallet.IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER)) {
        
        PartiallySignedTransaction psbtx(mtx);
        
        
        bool complete;
        wallet.FillPSBT(psbtx, complete, SIGHASH_ALL, false , true );
        const TransactionError err = wallet.FillPSBT(psbtx, complete, SIGHASH_ALL, true , false  );
        if (err != TransactionError::OK) return false;
        complete = FinalizeAndExtractPSBT(psbtx, mtx);
        return complete;
    } else {
        return wallet.SignTransaction(mtx);
    }
}
Result CommitTransaction(CWallet& wallet, const uint256& txid, CMutableTransaction&& mtx, std::vector<bilingual_str>& errors, uint256& bumped_txid)
{