I have a job working in a grocery shop during the year and was asked to come in during Chol Hamoed, which is a regular workday. May I do so?
Yes. It is permitted for you to continue to work as normal in a grocery shop over Chol Hamoed.
Although doing business and buying and selling is restricted during Chol Hamoed, it is permitted to be done for the sake of the Moed, and may be done even in public without any change if it is a product that everyone knows is being purchased for the sake of the Moed, such as fruits, vegetables, and other perishable foods which people go shopping for on an almost daily basis. This, however, only explains the permissibility for the grocery store owner to leave the shop open as usual during Chol Hamoed, however, it does not necessarily explain the allowance of the worker to work the year and earned money which is his own personal prohibition. Nonetheless, in truth anytime permitted a certain Melacha to be performed during the Moed due to reasons of need of the Moed, or need of the public, it may be performed even on behalf of payment, and hence the same way it is permitted for the owner to leave his grocery shop open it is permitted for his workers to work there and earn money.
Sources: See regarding the allowance to leave the grocery shop open over Chol Hamoed: Michaber 539:10; Piskeiy Teshuvos 539:6; See regarding collecting payment for permitted work: Michaber 542:1; 545:1; M”B 541:1; Biur Halacha 541:1 Afilu [that by Ochel Nefesh may collect money] and 544:1 “Tzarchei Rabim” [may collect money]; Chol Hamoed Kehilchaso 9:1-4