In fatwa number 319753 (http://www.islamweb.net/emainpage/index.php?page=showfatwa&Option=FatwaId&Id=319753), you did not clearly state whether the 15,000 rebate itself would be interest or not. Please have a look into the question again by entering the link above, and let me know this as soon as possible.
All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
We have already clarified in fatwa 319753, which you referred to, that the National Saving Scheme which you mentioned is a Riba (interest) scheme, so it is absolutely not permissible to join it, based on what we have come to know about the scheme, regardless of whether the subscriber is exempted from some taxes or not.
As to whether the rebate itself is considered interest, then this depends on whether the person has to pay legitimate taxes. So if he is exempted from the taxes when he subscribes to the usurious scheme, then this exemption is a kind of interest because this is included in the general rule which states that every conditional benefit in a loan is interest.
Allah knows best.
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