Without getting tax invoice can payment be made to agency as junior engineer are writing all the measurements in measurements book . it is self explanatory for GST deduction.
Is that supplier registered under gst or Not...???!
If registered then Yes, without Tax Invoice he can't give payment to agency.
If unregistered then Tax invoice not required or not possible to give tax invoice. Better get details through a letter head or contract assignment. After these suitable documents he can pay amount...