Pocket

Is there a plugin available in which customers can keep prepaid amounts or incase of order cancellation amount will be credited to customers account which can be used later in next purchase ?

Actually this is already built into opencart though it's not very obvious.

Log in to your admin, go to customer list, and find the customer you want to add a store credit to. Click edit, then Click on the "Transactions" tab and add a transaction (a bad name for issuing store credit but that is how it is listed).

When the customer checks out, any credits will automatically be applied. There is also a free mod by Q I think (for version 1.5 I know but probably also for v2) that will show a customer their available credit in their account page.

The one caveat is they have to be a registered customer (or you have to register them which I've done from time to time to issue someone a credit).