purchase exempt good from registered dealer but not get invoice and payment made by cheque

my query is :-

I am registered gst dealer and purchase exempt goods (atta non- branded) goods from registered dealer in may 2018 . but this dealer not give me invoice ( 2 lakh amt) . and I give payment via cheque . so what I do in my account . and if I show unregistered purchase so RCM is applicable in this ? too confusion