### 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_
LOCK(wallet.cs_wallet);
    if (!errors.empty()) {
        return Result::MISC_ERROR;
    }
    auto it = txid.IsNull() ? wallet.mapWallet.end() : wallet.mapWallet.find(txid);
    if (it == wallet.mapWallet.end()) {
        errors.push_back(Untranslated("Invalid or non-wallet transaction id"));
        return Result::MISC_ERROR;
    }
    const CWalletTx& oldWtx = it->second;
    
    Result result = PreconditionChecks(wallet, oldWtx,  false, errors);
    if (result != Result::OK) {
        return result;
    }
    
    CTransactionRef tx = MakeTransactionRef(std::move(mtx));
    mapValue_t mapValue = oldWtx.mapValue;
    mapValue["replaces_txid"] = oldWtx.GetHash().ToString();
    wallet.CommitTransaction(tx, std::move(mapValue), oldWtx.vOrderForm);
    
    bumped_txid = tx->GetHash();
    if (!wallet.MarkReplaced(oldWtx.GetHash(), bumped_txid)) {
        errors.push_back(Untranslated("Created new bumpfee transaction but could not mark the original transaction as replaced"));
    }
    return Result::OK;
}
} 
}