### 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 (solving_data) {
        args.push_back({"solving_data", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "Keys and scripts needed for producing a final transaction with a dummy signature.\n"
        "Used for fee estimation during coin selection.",
            {
                {
                    "pubkeys", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Public keys involved in this transaction.",
                    {
                        {"pubkey", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "A public key"},
                    }
                },
                {
                    "scripts", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Scripts involved in this transaction.",
                    {
                        {"script", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, "A script"},
                    }
                },
                {
                    "descriptors", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Descriptors that provide solving data for this transaction.",
                    {
                        {"descriptor", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A descriptor"},
                    }
                },
            }
        });
    }
    return args;
}
void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out, int& change_position, const UniValue& options, CCoinControl& coinControl, bool override_min_fee)