### 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 std::string& desc_str  = desc_univ.get_str();
                FlatSigningProvider desc_out;
                std::string error;
                std::vector<CScript> scripts_temp;
                std::unique_ptr<Descriptor> desc = Parse(desc_str, desc_out, error, true);
                if (!desc) {
                    throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Unable to parse descriptor '%s': %s", desc_str, error));
                }
                desc->Expand(0, desc_out, scripts_temp, desc_out);
                coinControl.m_external_provider.Merge(std::move(desc_out));
            }
        }
    }
    if (options.exists("input_weights")) {
        for (const UniValue& input : options["input_weights"].get_array().getValues()) {
            uint256 txid = ParseHashO(input, "txid");
            const UniValue& vout_v = input.find_value("vout");
            if (!vout_v.isNum()) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, missing vout key");
            }
            int vout = vout_v.getInt<int>();
            if (vout < 0) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout cannot be negative");
            }
            const UniValue& weight_v = input.find_value("weight");
            if (!weight_v.isNum()) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, missing weight key");
            }
            int64_t weight = weight_v.getInt<int64_t>();
            const int64_t min_input_weight = GetTransactionInputWeight(CTxIn());