Niang broke the deadlock for the visitors at the Stadium de Toulouse in the 35th minute with his ninth league goal of the season.
Five minutes later, Toulouse responded with an equaliser courtesy of Jimmy Durmaz's effort from the penalty spot.
After the restart, Togo’s Matthieu Dossevi gave the hosts the lead in the 51st minute but Sarr's effort from the spot seven minutes later ensured the teams shared the spoils.
Rennes dropped to 13th in the league standings with 45 points from 35 matches. They welcome Guingamp to the Roazhon Park for their next league match on May 12.