Sango was very persuasive before Avidius concerning his plan for the city and the desire to not kill him under the effect that he would help him out.
In order to prevent the downfall of more cities at the hands of Sango, Avidius had no choice but to hesitantly accept his deal.
Avidius said yes in exchange for his life and the lives of hundreds of thousands that would have died if Sango had decided to raid them.
Now that he has agreed, Avidius could no longer take aggressive actions toward Sango, not only because he accepted the deal, but also because he knew that there is nothing much he can do against him.
Although he couldn't take any aggressive means toward Sango, Avidius was not so willing to turn his back in Turdale city that fast.
Ever since he was spared by Sango, Avidius has been tagging alongside Sango to anywhere he is headed to.