석 or 서 is a colloquial form of "세"
like wise, 넉 or 너 is a colloquial form of "네"
According to th Korean language standardization:
Before the unit of "돈, 말, 발, 푼", 서 have to be used.
Before the unit of "냥, 되, 섬, 자", 석 have to be used.
The other cases, both 석 and 세 can be used.
서 돈(O) 세 돈(X)
석 냥(O) 세 냥(X)
석 잔(O) 세 잔(O)
석 달(O) 세 달(O)