Allow outputs to be increased

Author: NicolasDorier

bip-0078.mediawiki

@@ -271,10 +271,10 @@ The sender should check the payjoin proposal before signing it to prevent a mali
 *** The amount that was substracted from the output's value is less or equal to <code>maxadditionalfeecontribution</code>. Let's call this amount <code>actual contribution</code>.
 *** Make sure the actual contribution is only paying fee: The <code>actual contribution</code> is less or equals to the difference of absolute fee between the payjoin proposal and the original PSBT.
 *** Make sure the actual contribution is only paying for fee incurred by additional inputs: <code>actual contribution</code> is less or equals to <code>originalPSBTFeeRate * vsize(sender_input_type) * (count(original_psbt_inputs) - count(payjoin_proposal_inputs))</code>. (see [[#fee-output|Fee output]] section)
-** If the output is the payment output and payment output substitution is disabled.
+** If the output is the payment output and payment output substitution is allowed.
 *** Do not make any check
 ** Else
-*** Make sure the output's value did not changed.
+*** Make sure the output's value did not decreased.
 ** Verify that all sender's outputs (ie, all outputs except the output actually paid to the receiver) from the original PSBT are in the proposal.
 * Once the proposal is signed, if <code>minfeerate</code> was specified, check that the fee rate of the payjoin transaction is not less than this value.
 
@@ -561,8 +561,8 @@ public async Task<PSBT> RequestPayjoin(
             }
             else
             {
-                if (originalOutput.OriginalTxOut.Value != proposedPSBTOutput.Value)
-                    throw new PayjoinSenderException("The receiver changed one of our outputs");
+                if (originalOutput.OriginalTxOut.Value > proposedPSBTOutput.Value)
+                    throw new PayjoinSenderException("The receiver decreased the value of one of the outputs");
             }
             // We fill up information we had on the signed PSBT, so we can sign it.
             foreach (var hdKey in output.SignedPSBTOutput.HDKeyPaths)