The problems aren't primarily technical, its all been done before in some form. In theory you could have state owned banks open a new account for each "qualifying" citizen. These accounts will hold credits denominated in Euros but trade-able only in return for food items or taxes due.  Cards would be issued against each account. State wages or payments to suppliers would be paid in part to these new accounts, and in part in whatever is currently the usual way.

New accounting systems would have to be set up to account for the split payments at both Government procurement/HR level and employee/supplier level. Food shops and tax offices would have to receive terminals capable of settling bills from these new accounts.  Some anti-fraud audits would have to be conducted to prevent supemarkets accepting such payment for non-food items, but perhaps they would be less than keen to except non hard cash in the first place - especially if they received more in credits than they need to pay their taxes. Those who refuse to acept tax credits in payment at all might lose business or perhaps even a licence to trade.

Some companies who supply a lot of services to the Government would end up with more tax credits than the taxes they actually owe or are due to pay.  They will want to be able to trade tax credits for real cash.  Banks might be allowed to facilitate this trade provide the seller could prove they were fully tax compliant.

Companies with millions in cash stashed in Switzerland might have to draw on their cash hoard in order to pay their (especially foreign) suppliers as foreign suppliers won't accept tax credits - but receive only tax credits in part payment. Thus hard cash would slowly be leached out of its hides.  But all accounting systems would have to be able to deal with effectively two currencies being used to settle single bills - not a usual IT requirement.  Probably some bills would be settled in hard currency, some in tax credits but many suppliers would be unhappy if they already had enough tax credits to settle their tax liabilities.  Some might only accept payment in tax credits for a discount, and so a grey market might be born.

There are many precedents for businesses operated in two currencies - some N. Ireland shops accept Euros as well as Sterling often at a standard notional trading rate. such systems should be adaptable to running a parallel currency, but for simplicity I suggest an official 1:1 parity should always be maintained, whatever happens on the grey market.

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