Implement disable_output_substitution check correctly

When disable_output_substitition is true, the receiver may not change
the script pubkey or decrease the value of their original output, but is
allowed to increase the amount or add new outputs.

payjoin/src/receive/mod.rs

@@ -360,8 +360,22 @@ impl WantsOutputs {
         match outputs {
             Some(o) => {
                 if self.params.disable_output_substitution {
-                    // TODO: only fail if the original output's amount decreased or its script pubkey is not in `outputs`
-                    return Err(Error::Server("Output substitution is disabled.".into()));
+                    // Receiver may not change the script pubkey or decrease the value of the
+                    // their output, but can still increase the amount or add new outputs.
+                    // https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#payment-output-substitution
+                    let original_output = &self.psbt.unsigned_tx.output[self.owned_vouts[0]];
+                    match o.iter().find(|txo| txo.script_pubkey == original_output.script_pubkey) {
+                        Some(txo) if txo.value < original_output.value => {
+                            return Err(Error::Server(
+                                "Decreasing the receiver output value is not allowed".into(),
+                            ));
+                        }
+                        None =>
+                            return Err(Error::Server(
+                                "Changing the receiver output script pubkey is not allowed".into(),
+                            )),
+                        _ => log::info!("Receiver is augmenting outputs"),
+                    }
                 }
                 let mut replacement_outputs = o.into_iter();
                 let mut outputs = vec![];