Q: If the government requires turning in lost items to police regardless of the owner or circumstances of the lost item, is that also a halachic requirement?

A: There is a halachic principle that dina d’malchusa dina, the law of the land has validity. However, there is significant discussion among the authorities about when this rule applies (C.M. 369:6-10).

Regarding treasures lost at sea, which one is permitted to keep, the Rema writes that if the king or beis din instituted that they must be returned, one must do so. The Rema writes a similar rule regarding returning a stolen item, even after the owner abandoned hope of reclaiming it (C.M. 259:7; 356:7).

This seemingly indicates that the rule of dina d’malchusa applies also to hashavas aveidah. However, some Acharonim write that between individuals, dina d’malchusa applies only when there is also an institution of beis din to this effect; then the law becomes common practice. Or it applies between individuals where there is an ethical recommendation (lifnim mishuras hadin) to return the lost item. Where there is absolutely no halachic basis to return an item, the rule of dina d’malchusa does not apply (see Shach 356:10; Ketzos 259:3).