Is it permissible to accept a sum less than the phone's value as compensation for it, after a friend claimed to have returned it and the questioner denied that?

If he has no proof that he returned the mobile phone, and you are certain that he did not return it, then he must return a similar one or pay its value, as it is a guaranteed trust. It is permissible for both of you to reconcile on the basis that he pays less than the phone's value. Scholars have unanimously agreed on the permissibility of reconciliation, based on the Almighty's saying: "And if two factions among the believers should fight, then make settlement between the two," and on the Prophet's (peace be upon him) saying: "Reconciliation is permissible among Muslims, except for a reconciliation that makes a lawful thing unlawful or an unlawful thing lawful."