### 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_
const UniValue& recipient{recipients[i]};
                if (recipient.isStr()) {
                    UniValue rkvp(UniValue::VOBJ);
                    rkvp.pushKV(recipient.get_str(), 0);
                    recipient_key_value_pairs.push_back(rkvp);
                    addresses_without_amount.insert(recipient.get_str());
                } else {
                    recipient_key_value_pairs.push_back(recipient);
                }
            }
            if (addresses_without_amount.size() == 0) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Must provide at least one address without a specified amount");
            }
            CCoinControl coin_control;
            SetFeeEstimateMode(*pwallet, coin_control, options["conf_target"], options["estimate_mode"], options["fee_rate"], false);
            coin_control.fAllowWatchOnly = ParseIncludeWatchonly(options["include_watching"], *pwallet);
            if (options.exists("minconf")) {
                if (options["minconf"].getInt<int>() < 0)
                {
                    throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid minconf (minconf cannot be negative): %s", options["minconf"].getInt<int>()));
                }
                coin_control.m_min_depth = options["minconf"].getInt<int>();
            }
            if (options.exists("maxconf")) {
                coin_control.m_max_depth = options["maxconf"].getInt<int>();
                if (coin_control.m_max_depth < coin_control.m_min_depth) {
                    throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("maxconf can't be lower than minconf: %d < %d", coin_control.m_max_depth, coin_control.m_min_depth));
                }
            }
            const bool rbf{options.exists("replaceable") ? options["replaceable"].get_bool() : pwallet->m_signal_rbf};