‘And Jacob came from the open country in the evening and Leah went out to meet him and said, “You must come in to me for I have surely hired you (‘sachar' - to hire for wages) with my son's mandrakes.” And he lay with her that night. And God listened to Leah and she conceived and bore Jacob a fifth son. And Leah said, “God has given me my hire (sachar) because I gave my handmaid to my husband.” And she called his name Is-sachar (hired man).'
Leah clearly has a sense of humour. Personally she sees the name as resulting from her hiring of Jacob with the mandrakes, but in God's eyes and in the eyes of others she sees it as her reward for allowing her handmaid to bear children on her behalf.
“The open country” or ‘field'. It may well be that he had been labouring in the wheat fields where Reuben had found the mandrakes.