When you sell 100 shares of Reliance, the 20% of sale proceeds will be blocked (this will be available the next day), the 80% which you will get can be used for other trading purposes, you can also use the sale proceeds received to buy back the Reliance shares again, but as you have received only 80% of the sale value if this are the only funds in your account you will be able to buyback only 80% shares. If you have additional funds in your account you can buy back 100% shares sold.
Also, you cannot use collateral margin received from pledging securities to purchase shares, you will need free cash for this.