The purchaser may increase the amount of the advance payment above 30% of the amount calculated in accordance with the provisions of article R. 2391-4. In this case, it may make payment conditional upon the provision of a first demand guarantee. This guarantee may cover all or part of the advance payment. The two parties may agree to substitute this guarantee with a personal and joint guarantee. The provision of this guarantee may not, however, be required of public entities holding a contract.