### 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_
},
                    },
                    {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                    {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                    {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                    {"subtract_fee_from_outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Outputs to subtract the fee from, specified as integer indices.\n"
                    "The fee will be equally deducted from the amount of each specified output.\n"
                    "Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                    "If no outputs are specified here, the sender pays the fee.",
                        {
                            {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                        },
                    },
                },
                FundTxDoc()),
                RPCArgOptions{.oneline_description="options"}},
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                    {RPCResult::Type::STR_HEX, "txid", true, "The transaction id for the send. Only 1 transaction is created regardless of the number of addresses."},
                    {RPCResult::Type::STR_HEX, "hex", true, "If add_to_wallet is false, the hex-encoded raw transaction with signature(s)"},
                    {RPCResult::Type::STR, "psbt", true, "If more signatures are needed, or if add_to_wallet is false, the base64-encoded (partially) signed transaction"}
                }
        },
        RPCExamples{""
        "\nSend 0.1 BTC with a confirmation target of 6 blocks in economical fee estimate mode\n"
        + HelpExampleCli("send", "'{\"" + EXAMPLE_ADDRESS[0] + "\": 0.1}' 6 economical\n") +
        "Send 0.2 BTC with a fee rate of 1.1 " + CURRENCY_ATOM + "/vB using positional arguments\n"