What is the ruling on selling infant formula purchased with "food stamps" back to the store at a lower price?

It is not permissible to engage in buying or selling with someone known to intend to defraud the state regarding items it has granted on condition of need, because the jurisprudential texts indicate the obligation to adhere to the condition of the granter or endower, provided it does not contravene Islamic law. As for someone who bought an item for their need and then wished to sell or exchange it without fraudulent intent, there is no harm in dealing with them. The same applies to someone whose situation is unknown, as the default is permissibility. And Allah knows best.