Gst impact on services to sez units

One of Our client is located in SEZ. We provide housekeeping services to them. While billing to them for our servies they told us to do following :

• Charge IGST in Bill @ 18%
• We will have to pay IGST to Govt and then we can claim for Refund
• They will pay to us only service amount in bill i.e. excluding IGST

is this a correct procedure? if not then please advise.