Use static_cast when calculating donation and add some debugging

src/transaction_builder.cpp

@@ -54,6 +54,9 @@ void TransactionBuilder::AddSaplingOutput(
     CAmount value,
     std::array<unsigned char, HUSH_MEMO_SIZE> memo)
 {
+    if(fZdebug) {
+        LogPrintf("%s: adding output with value=%ld\n", __func__, value);
+    }
     auto note = libzcash::SaplingNote(to, value);
     outputs.emplace_back(ovk, note, memo);
     mtx.valueBalance -= value;