If offset was already saved for a partition, sarama-cluster will resume consumption from that offset. The
Config.Producer.Offsets.Initial option is used only if no saved offset is present (first run for a consumer group).
You can verify this by adding the following line at the beginning of your
sarama.Logger = log.New(os.Stdout, "sarama: ", log.LstdFlags)
Then you'll see something like the following in the output:
cluster/consumer CID-17db1be4-a162-411c-a106-4d198191176a consume sample/0 from 12
The 12 in that is the offset Sarama is going to start from for that partition (sample/0).