There shall be submitted to the voters the question whether or not such city shall incur an indebtedness and evidence the same by the issuance of bonds for the purpose of providing funds to pay by way of assistance to the United States, or its qualified authority, in consideration of and in order to induce the location and establishment within such city of such park or plaza, the sum fixed by the ordinance providing for the submission of the question, which sum shall not exceed one-fourth of the total amount to be expended by the United States, or its authority, for the acquisition and establishment of such park or plaza (including site and improvements), and shall not exceed the sum of eight million dollars; nor shall such sum, when added to the other indebtedness of the city, exceed its capacity to become indebted under the constitution of this state.