### 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_
RPCHelpMan psbtbumpfee() { return bumpfee_helper("psbtbumpfee"); }
RPCHelpMan send()
{
    return RPCHelpMan{"send",
        "\nEXPERIMENTAL warning: this call may be changed in future releases.\n"
        "\nSend a transaction.\n",
        {
            {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "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"
                    "For convenience, a dictionary, which holds the key-value pairs directly, is also accepted.",
                OutputsDoc(),
                RPCArgOptions{.skip_type_check = true}},
            {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},