### 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 (!fee_rate.isNull()) {
        if (!conf_target.isNull()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both conf_target and fee_rate. Please provide either a confirmation target in blocks for automatic fee estimation, or an explicit fee rate.");
        }
        if (!estimate_mode.isNull() && estimate_mode.get_str() != "unset") {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both estimate_mode and fee_rate");
        }
        
        cc.m_feerate = CFeeRate{AmountFromValue(fee_rate, 3)};
        if (override_min_fee) cc.fOverrideFeeRate = true;
        
        if (!cc.m_signal_bip125_rbf) cc.m_signal_bip125_rbf = true;
        return;
    }
    if (!estimate_mode.isNull() && !FeeModeFromString(estimate_mode.get_str(), cc.m_fee_mode)) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, InvalidEstimateModeErrorMessage());
    }
    if (!conf_target.isNull()) {
        cc.m_confirm_target = ParseConfirmTarget(conf_target, wallet.chain().estimateMaxBlocks());
    }
}
RPCHelpMan sendtoaddress()