Welcome to the wonderful country of Argentina my fellow compatriot!
I too had this pleasant conundrum and I walked out the bank with a backpack full of my life savings. It was a wonderful walk down centro to reflect on my life choices and all the modern marvels that Argentina has to offer (sarcasm intended).
You have two options...say yes to the seller, or say no. It's as simple as that. Personally, I find this "standard practice" of cash-only a disgusting habit worthy of the lowliest mafia bookie. You can definitely request a transfer only and tell the seller to shove it but be prepared for the deal to fall apart or for the additional fees to be added to the purchase price. Evading taxes in this country is a national sport.
That being said, understand that it's considered "mutually beneficial" to pay at least some portion in cash. A wire would imply 100% en blanco. The full purchase price is rarely declared on the compra/venta which allows you to "save" on your closing costs and for them to reduce taxes. It's all a "wink wink" lie and you don't have to be complicit if you don't wish to. It's not however always avoidable.
If you have to get cash... have a trusted car waiting for you right outside the bank so you go straight from the bank door to the car door (10 feet). The car will take you directly to the front of the escribano's office where you will go straight from the car door to the office door (10 feet). I would also recommend having an extra friend to accompany you to the cashier and to the car. It is a stressful moment to see that much cash and it helps to have someone there to ensure everybody is paying attention to the withdrawal (you included). Let the bank know a few days in advance you will withdraw all your dollars and ask if they have the proper bills on hand. New issue blue bills, no cara chica/grande. It's a lot of bills and you will definitely not have a chance to review every single one of them for scratches, tears or stains. Do not tell the bank exactly what time you will be there to withdraw the cash. Some cashier's may tip off an unsavory friend.
God speed and congratulations.
