It is not a problem of Ribis, since it does not involve an increased price for a delay in payment. However, you are correct that there is an issue of Dina d'Malchusa Dina involved. When I asked Rav Moshe Sternbuch this question (about a merchant requesting cash and discounting the payment by the V.A.T. (in Israel, the Value Added Tax, which is %17), he said that the practice is so widespread to pay in cash and avoid the VAT that the government is not Makpid about it, and he therefore permitted it. (In Israel, it is the merchant's responsibility to pay the tax, and thus to collect the VAT from the customer. It is not the customer's responsibility to pay the VAT per se to the merchant. That is, if the merchant wants to report his income to the government, he will have to pay the tax even if he gave a discount to the customer. Based on this, another Posek permitted the customer to pay in cash since it involves various Sfeikos of Dinim d'Rabanan of Lifnei Iver.) A friend of mine heard from Rav Elyashev, though, that although it is not Asur per se to pay in cash so that the merchant can avoid paying the tax, it is also not Asur to be Machmir in this regard. In Rav Elyashev's words (translated to English), "If you get to Olam ha'Ba and the only Aveirah that you have in your hands is that you paid taxes to a Zionist government, you won't be in bad shape."

The law in other countries (where they are Makpid on VAT) might be different. For practical Halachah, consult with a competent Halachic authority.

Y. Shaw