Is breaking an oath by not performing mandatory overtime work, then reaching an understanding and performing the work, considered an "oath that plunges one into sin" (yamin ghamus) and does it necessitate expiation?

The yamin ghamus (perjurious oath) is a deliberately false oath that plunges its maker into sin and Hellfire. The oath mentioned is not a ghamus, but rather a binding oath that necessitates expiation upon breaking it. There is no harm in breaking the oath if one believes something else is better. The questioner is therefore liable for an expiation for an oath, as Allah Almighty says: "Its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever finds not [the means to do so] - then a fast of three days." (Al-Ma'idah: 89).