As per Sec 9(4) of CGST Act, if a registered person purchases goods/services from an unregistered dealer (URD) then the registered taxpayer is liable to pay GST on reverse charge basis( only for certain goods/services & registered persons).
Oh sorry, I think you can claim ITC even if you make a sale to an unregistered dealer as you will be charging him GST on the sale no matter he is registered or not. this is not fall in Rcm