No expenditure shall be made from the street improvement revolving fund for a purpose which would not be legal under the accounts from which the component parts of the fund were taken. Such expenditures shall be limited to making advance payments for deposits as required by law in order to make available to the City federal highway aid grants, rights-of-way or other incidental expenses incurred in road improvement work, which shall be later paid back into the street improvement revolving fund from motor vehicle fuel tax money. (Code 1962, § 35-15)