The considered opinion here is that a clause in rental contracts that provides rent must be paid in foreign currency is voidable and unenforceable. So, for example, if you are in possession and occupying an apt pursuant to a lease that provides for payment in US $, you can refuse to pay and require that the owner accept pesos (at the official rate of exchange) in lieu of the dollar price. He will not be able to evict you if you offer the correct legal rent. Accordingly, you may want to do to your Ar landlord what they have been trying to do their tenants (screw them) - agree to pay in dollars, move in, and then insist upon your legal right to pay in pesos regardless of the signed lease term to the contrary.
If you are not in possession (occupying) the apt, chances are the owner will either refuse to accept pesos or set the peso rent at a higher rate (equivalent to the cost in US$ x the black market peso price). He may also want the right to raise the peso price periodically if the term of the lease is more than a couple of months.
Temporary renting of apts is fraught with pitfalls, e.g., difficulty of getting deposit back, (illegal) demand to pay in foreign currency, refusal to make necessary repairs, etc. Try to protect yourself by setting some clear terms about these things.