Tax payment on advance in gst for work contractor

I am having GST number works for contractor as a sub contractor of POP work. Gave me 2 lacs advance but work will do long. should I pay tax on advance. but buyer(contractor)says no need to pay tax on advance, pay only when you raise invoice. no matter how much amount credited into your bank account. for example : in the month of may18 he gave me 5 lacs advance through RTGS. never pay tax file Nil. when you raise invoice in aug 18 then pay tax. is this right. how to do. pls help.