There are stores in some of the legalized states that are processing debit card transactions through some company in NYC. I expect it will be possible in MA stores.
Essentially, to the bank the debit card transactions look like they're coming from an ATM. That 'ATM' is owned by a wholly owned subsidiary of the dispensary that, IDK, ostensibly sells t shirts or something. Then the 'cash' out of the 'ATM' is immediately used in your transaction.
There's no way the banks involved don't know what's going on, but there's just enough plausible deniability that they're going with it for the moment. But all it takes is one dropping the hammer on one instance of this and everyone will be back to pure cash.