Hello devrant,
Need guidance about QR code based coupon system.

In my restaurant I sale few food items. My 50+ customer bought food daily (on delivery). They are asking me for coupon system like I sale one sandwich for half 30 INR , they bought 3-4 per day.
So they can buy coupon of 100 sandwich on every delivery my delivery boy will scan QR code of customer and their sandwich count will decrease accordingly.

This will apply for every food item.

Can you suggest me current software for this.

A QR is text.
It is usually used to represent a link to a web page.
What you (seem) to want is:
A web service accessible by anyone with the link and a function that takes in input a string (which will be printed inside the QR e.g.: http://yourqrservice.com/redeem/...]).
All QR will need to be single use or have a limited number of times.
Then if the QR can be used multiple times or if it is stored in a place where people can take a picture of it you will want to implement:
1. A registration and authentication system for your users.
2. An assignment system where users who own a QR code can transfer it to another user.
3. All unassigned QR codes are generated as assigned to someone.

You need this because if I manage to take a picture of the QR I can use it even if I do not have the original QR. When given to someone the QR will be assigned by using the web page.

Alternatively just setup an account for each QR, it will still need a password and to enter the quantities.