### 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_
"original transaction that were less than 0xfffffffe will be increased to 0xfffffffe\n"
                             "so the new transaction will not be explicitly bip-125 replaceable (though it may\n"
                             "still be replaceable in practice, for example if it has unconfirmed ancestors which\n"
                             "are replaceable).\n"},
                    {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
                             "\"" + FeeModes("\"\n\"") + "\""},
                    {"outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "The outputs specified as key-value pairs.\n"
                             "Each key may only appear once, i.e. there can only be one 'data' output, and no address may be duplicated.\n"
                             "At least one output of either type must be specified.\n"
                             "Cannot be provided if 'reduce_output' is specified.",
                        OutputsDoc(),
                        RPCArgOptions{.skip_type_check = true}},
                    {"reduce_output", RPCArg::Type::NUM, RPCArg::DefaultHint{"not set, detect change automatically"}, "The 0-based index of the output from which the additional fees will be deducted. In general, this should be the position of change output. Cannot be provided if 'outputs' is specified."},
                },
                RPCArgOptions{.oneline_description="options"}},
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "", Cat(
                want_psbt ?
                std::vector<RPCResult>{{RPCResult::Type::STR, "psbt", "The base64-encoded unsigned PSBT of the new transaction."}} :
                std::vector<RPCResult>{{RPCResult::Type::STR_HEX, "txid", "The id of the new transaction."}},
            {
                {RPCResult::Type::STR_AMOUNT, "origfee", "The fee of the replaced transaction."},
                {RPCResult::Type::STR_AMOUNT, "fee", "The fee of the new transaction."},
                {RPCResult::Type::ARR, "errors", "Errors encountered during processing (may be empty).",
                {
                    {RPCResult::Type::STR, "", ""},
                }},
            })
        },