Belgium-based Sakala scored after seven and 89 minutes at the National Heroes Stadium in Lusaka while Jacques Temopele netted for Congo during the opening minute of the second half.
Sakala, who plays for Belgian top-flight club KV Oostende, has scored all four Zambian goals in qualifying having netted the home and away winners against Malawi last March.
The 22-year-old forward was part of the Zambian side that won the 2017 African youth championship with a 2-0 victory over Senegal in the final in Lusaka.
He made a disastrous debut for the senior team the same year, getting sent off after two yellow cards in a World Cup qualifier against Algeria.
Zambia and Congo meet again Sunday and the aggregate winners will become the first nation to secure a place at a November 8-22 Olympic Games qualifying tournament in Egypt.
Nigeria and Cameroon, who won the 1996 and 2000 Olympic football tournaments respectively, have first leg fixtures later this week.
The Nigerians play Sudan in Omdurman Thursday and the Cameroonians host Tunisia in Yaounde the following day.
South Africa, Zimbabwe, Ghana, Algeria, Ivory Coast, Guinea, Morocco and Mali are also contenders for places at the tournament in Egypt, from which three countries will secure places in Japan.
Olympic qualifying is restricted to under-23 footballers with each country permitted three "wild cards" at the Games.