Is it permissible to give as charity money from a plug that has not yet been returned to its owners, and would this absolve one's responsibility, and for whom would the reward be in the case of giving such charity?

If the plug (i.e., the specific item) is equivalent to or less than the right of the claimant, then there is no harm in taking it and benefiting from it, based on the opinion of Imam Al-Qurtubi, who permits taking what is equivalent to the right. This is the sounder of the two opinions for Al-Shafi'i, by analogy with obtaining the exact type of property. However, since the claimant still has doubt, the advice is to return the plug to the estate, even through indirect means, to reassure oneself. This is not obligatory for him if he dies before returning it. Giving charity does not remove doubt; nevertheless, he will be rewarded for the charity if he performs it.