mode of receiving payment

as I being dealer and selling my goods to retailers who are not registered nor having any bank involvement.....they accept payments from consumer as cash...and whatever they get cash from consumer ..they pay cash against the GST bill issued by me for the goods...m I doing it correctly or do I need to accept only cheque or online payment from the retailer....
example....I sell goods worth 3 lakh to retailer ...and retailer give this 3 lakh in part viz...30k,1lakh,50k,70k,50k on different dates after selling goods to last consumer...and they receive payments from consumer as cash....