Is it obligatory to guarantee the phones of a foolish person that were gifted five years ago and have now been lost, and how should their price be determined: at the time of the gifting or now, taking into consideration the change in phone prices and currency value?

The "safih" (prodigal/financially incompetent) is someone who does not manage money well, or who squanders their wealth on permissible or impermissible desires. This includes those who buy at more than the fair value or sell at less than it without justification.

If the safih has reached puberty, their transactions are not valid unless approved by their guardian. Anything they have given must be returned unless their guardian approves it. However, if they reached maturity and then became safih, and no interdiction was placed upon them, their transactions are valid.

If something given by the safih must be returned but is lost, its equivalent must be returned. If an equivalent is not found, then its value at the time it was taken must be returned. It is preferable to return something close to its equivalent if available.