Although I'm sure there will be people these days who would disagree with this, the value of a currency is based upon the value of goods and services you can receive for that currency. An exception being the rare precious metal currency but no one uses those these days.

If a country has basic income, it should be clear by now that productivity will significantly decrease.

Aside from what this would mean domestically, the fact that people in a basic income country are forcing people to sell and buy things for monopoly money would not mean that foreigners in a global supply chain can expect to get good value out of a currency that is connected to a basic income system. Since no one can force them to accept said monopoly money as payment, even if people domestically were forced to use it, it would devalue at a faster rate internationally than it would domestically. This would eventually have a ripple effect back to the country attempting UBI, even in an environment where they have successfully forced their own citizens to use the devalueled currency.