Saint-Etienne edged Rennes 1-0 at Roazhon Park on Thursday to leapfrog their hosts in the Ligue 1 table and move up to fifth.
Christophe Galtier's men had not won away in the league since September but were on top for long spells against a Rennes side hammered 4-0 by Bordeaux at the weekend.
Kevin Monnet-Paquet had the first half's only shot on target 21 minutes in and Saint-Etienne remained the more dangerous after the break.
Rennes - still finding their feet under new boss Rolland Courbis - were unable to muster an effort on target in the second half either as Sall's header wrapped up the points 17 minutes from time.
The Senegal defender registered his third goal of the campaign from Renaud Cohade's delivery, climbing above Romain Danze to ensure his side a first win at Rennes since December 2005.