Is it permissible to donate a monthly sum for stolen goods that are difficult to return to their owners, or is it obligatory to seek their forgiveness?

For a sincere repentance from theft, you must return the stolen goods to their owners as much as possible. If you are unable to identify their owners after a diligent search, then donate the stolen goods or their value as charity. When you find their owners, give them the choice between reclaiming their right or accepting it as charity on their behalf. Increase your good deeds, for the owners of rights may demand them from you on the Day of Judgment.