under Income Tax Act a person can receive in cash from a single person in a day against and invoice up to Rs.199999. Doesn't matter the purchaser is registered under GST or not.
Yes, you can receive and do whatever you want to do with cash. There is no legal problem for receiving payment within Rs. 2,00,000/- (i.e. upto 1,99,999/-).
If you receive amount of Rs. 2,00,000 or more in cash then that would be infringement of Sec 269ST and then 100% of Such amount received would be charged as Penalty u/s 271DA.