You can do it through the Kits config.
In oxide/config/Kits.json (or carbon/config/Kits.json) set your only currency to the banknote item:
"Economies": {
"0": {
"Type": "Item",
"ShortName": "sticks",
"Skin": 2536195910,
"DisplayName": "Bank Note"
}
},
"DefaultCurrency": 0
In oxide/config/Kits.json (or carbon/config/Kits.json) set each purchasable kit to use that currency and a price, for example:
"MyKit": {
"Sale": true,
"Price": 50,
"DefaultCurrency": 0
}
Then save the files, reload the plugin (o.reload Kits / c.reload Kits), and test buying the kit with only those banknotes in your inventory.