Is it obligatory for the giver to inform the recipient that the money is from forbidden interest, or for the one giving zakat to inform the recipient that the money is zakat money?

It is not obligatory to inform the poor person that the money given to him is from usurious interest, even if he is poor. This money is considered among the expenditures of illicit funds whose owner is unknown, and he is not required to give it in charity again. Similarly, it is not obligatory to inform him that the money is Zakat, unless he abstains from taking Zakat; in that case, it is obligatory to inform him so that what he does not desire does not enter his possession.