It's legal for a landlord to demand that rent be paid in cash or on a cash basis, according to Avvo. Cash serves as "legal tender" in the United States, so all contracts may be paid that way. Some leases permit payment with a check; these terms are typically negotiable.
However, without the provision, the landlord is allowed to mandate payment in any reasonable form. One method that provides funds on a cash basis, while also giving the tenant a receipt, is a money order. In this case, it's reasonable to have the landlord credit the money order fee when paying rent. The cost is generally quite low, so it's also worth considering just paying it, notes Avvo.