Every retailer maintaining a place of business in this City who is authorized by the Director of Finance to collect the tax imposed by this article and who makes sales of tangible personal property for use or other consumption in this City not exempted under the provisions of this article, shall collect the tax from the purchaser at the time of making the sale, or, if the use or other consumption of the tangible personal property is not then taxable hereunder, at the time the use or other consumption becomes taxable. Upon collecting the tax the retailer, on demand, shall give to the purchaser a receipt therefor in the manner and form prescribed by the Director of Finance. In all cases in which the tax is not collected by the retailer as aforesaid, the person upon whom such tax is imposed shall pay the same when due to the Director of Finance or his authorized agent. The tax so collected by the retailer constitutes a debt owed by the retailer to the City and it shall be unlawful for such retailer to fail to pay the same to the City in the manner and at the times elsewhere provided in this article.