Starting from a judgement of the Court of Cassation, the contribution focuses on some problematic aspects related to the soluti retentio governed by art. 2035 of the Italian Civil Code. In the frequent cases in which the contract is, at the same time, contrary to mandatory rules and immoral, the general clause of good morals (buon costume) seems to be overwhelmed by considerations of a juridical nature able to explain the function of the institute and its scope of application. The main point of reference is the purpose of the mandatory rule infringed by the contracting parties, which the denial of restitution can help to achieve.