### 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_
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    LOCK(pwallet->cs_wallet);
    CAmount nAmount = AmountFromValue(request.params[0]);
    CFeeRate tx_fee_rate(nAmount, 1000);
    CFeeRate max_tx_fee_rate(pwallet->m_default_max_tx_fee, 1000);
    if (tx_fee_rate == CFeeRate(0)) {
        
    } else if (tx_fee_rate < pwallet->chain().relayMinFee()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("txfee cannot be less than min relay tx fee (%s)", pwallet->chain().relayMinFee().ToString()));
    } else if (tx_fee_rate < pwallet->m_min_fee) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("txfee cannot be less than wallet min fee (%s)", pwallet->m_min_fee.ToString()));
    } else if (tx_fee_rate > max_tx_fee_rate) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("txfee cannot be more than wallet max tx fee (%s)", max_tx_fee_rate.ToString()));
    }
    pwallet->m_pay_tx_fee = tx_fee_rate;
    return true;
},
    };
}
static std::vector<RPCArg> FundTxDoc(bool solving_data = true)
{
    std::vector<RPCArg> args = {
        {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks", RPCArgOptions{.also_positional = true}},
        {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
         "\"" + FeeModes("\"\n\"") + "\"", RPCArgOptions{.also_positional = true}},
        {
            "replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Marks this transaction as BIP125-replaceable.\n"
            "Allows this transaction to be replaced by a transaction with higher fees"