I obviously can't speak to Serbian tax law, but this rule (invoice =income) is applied universally. In my country if the invoice is not paid then that becomes

I obviously can't speak to Serbian tax law, but this rule (invoice =income) is applied universally. In my country if the invoice is not paid then that becomes a "bad debt", which in accounting terms is an expense (which in effect is then a tax credit.) There is sometimes a tax approach for very small companies where tax is calculated purely from invoice, and not from net-income, but there are typically benefits to that which outweigh the bad debts.