The German marque's competition director responded to news that Ferrari, unlike any other team, intend to test in the days prior to the Nurburgring.
"I don't want to always be critical," said Haug, allied to Ron Dennis' McLaren outfit.
The comments come at a time when the 'group of nine' coalition, reportedly unwilling to accept Ferrari's all-out approach for much longer, may be on the verge of collapse.
Haug told RTL television: "Whether (Ferrari) join us, whether they do not, it is up to them.
"We certainly invite them to once again consider the agreement."