Oyongo, obtained Jan. 27 in a trade with the New York Red Bulls, has reportedly claimed to be a free agent because he had amateur status when he was loaned to New York last season from Rainbow FC Bamenda of Cameroon.
Then the Associated Press reported that the Red Bulls had bought Oyongo's contract "on a permanent basis" from Bamenda. But late Thursday, there was no such announcement on the team's website.
A statement from the Impact said the club will do all it can to make sure he reports to Montreal.
"We have gone step by step with the league on this matter," the statement said. "His contract is in accordance with FIFA regulations and we will take all necessary actions to make sure that the contract is respected."
Oyongo was competing in the Africa Cup of Nations when the trade went through.
His agent, Nicolas Onisse, told France Football that the trade was "unfair, not right and incomprehensible. To transfer a player without warning, without asking how he feels, is a big problem in MLS."
Trading players is rare in sports outside North America.
Reports said the Cameroon soccer federation ruled that because it considered Oyongo to have amateur status, his professional contract was invalid and he should have been considered a free agent when the trade was completed.
Oyongo was on loan to the Red Bulls when he played 13 games last season, recording three assists.
The Cameroon international is described as a quick-footed left back who is strong both on defence and joining the attack. He was reported to be on the radar of several European squads, including Glasgow Celtic.
The Impact acquired Oyongo, midfielder Eric Alexander, allocation money and an international roster spot from the Red Bulls for midfielder Felipe Martins and the top pick in the allocation order. New York used the allocation order spot to sign U.S. international Sacha Kljestan from Belgian club Anderlecht.
The allocation order is used to distribute U.S. national team players who want to return to MLS.
This week, Impact coach Frank Klopas said Oyongo had not reported because he was resting and spending time with his family after the African Cup.