PaymentVariantsWindow: Warn user about negative credit exports

Elwig/Models/Dtos/Transaction.cs

@@ -15,7 +15,7 @@ namespace Elwig.Models.Dtos {
 
         public static IEnumerable<Transaction> FromPaymentVariant(PaymentVar variant) {
             return variant.Credits
-                .Where(c => c.Member.Iban != null)
+                .Where(c => c.Member.Iban != null && c.Amount > 0)
                 .OrderBy(c => c.TgNr)
                 .Select(c => new Transaction(c))
                 .ToList();