Review WASN'T GIVEN BACK CHANGE

I boarded a matatu from Ngara at around 5:50pm and alighted at Kenyatta road some minutes to 6:50pm.
I gave the conductor Ksh 500 and didn't receive back my change. He said he didn't have the change when I asked for it and I only remembered after alighting.
How should I follow this up? The fare was Ksh 70.