By definition, a reimbursement reimburses you for the cost of something that you bought, because you were required to buy it. A reimbursed expense is not taxable income. If you return the item and get cash for it, then it did not cost you anything, yet in the scenario the OP is inquiring about, you are claiming an expense that you do not have. The timing of the return is not relevant. Also, you are creating an accounting nightmare. Actually, not a nightmare, since there is IMHO no ambiguity. The money you would receive as a reimbursement for a purchase that is returned becomes taxable income. The MSC should then include it on a 1099 form, not exclude if because it is a reimbursement.
So - I think delving into the tax ramifications settles the issue. Unless you are willing to tell the MSC that you returned the item for cash and took a reimbursement and converted it into taxable income, you can't return an item for which you are reimbursed.
Ah, you say, I don't make enough to get a 1099. Doesn't matter, The MSC aggregates all reimbursements and reports them on their income tax. They do the same for payments. And since you are required to report all income even if you don't get a 1099, you have to pay income tax on the money which the MSC sent you as a reimbursement but which you converted into income.
A reimbursement is actually worth more than face value since you don't pay income tax or FICA on it. So, maybe a $1,00 is worth $1.50 depending upon your tax rate. If you convert it into income, it is worth about $0.50 (again, it depends upon your tax rate). So, to gain $0.50, you have given away $1.00, not mention that I think it is against the rules of virtually every MSC I do shops for.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008