Merits and Karma are just like the Fixed Deposit ?FD? and Saving Account in Banking system. For FD?our monies are locked, whereas in Saving Account, we can use the money any time.
Whether our action will collect merits or karma, it depends on our motivation. If we doing an action for the benefits of others, then we are collecting merits. If we are doing a good deed for the benefit of ourself, then we gain good karma.