To meet obligations incurred pursuant to appropriations for any fiscal year, the legislature may by law authorize the state to issue its full faith and credit notes in which case it shall pledge undedicated revenues to be received within the same fiscal year for the repayment thereof. Such indebtedness in any fiscal year shall not exceed 15 percent of undedicated revenues received by the state during the preceding fiscal year and such debts shall be repaid at the time the revenues so pledged are received, but not later than the end of the same fiscal year.