Edit : sorry I misunderstood it.. merit counter means your total merit in your profile? if that's what it meant then my answer is it does not change.
Yes, you are right. 
Your earned Merit (for you 543 Merit) will never decrease because it isn't affected when you are meriting another user's post. Sending Merit does only mean that you'll need 1 sMerit (sendable Merit) to do so, the amount of your sMerit is displayed directly after you click +Merit for a post you want to give Merit to.
You'll see this statement:
Quote
You have received a total of 1648 merit. This is what determines your forum rank. You typically cannot lose this merit. You have 5 sendable merit (sMerit) which you can send to other people. There is no point in hoarding sMerit; keeping it yourself does not benefit you, and we reserve the right to decay unused sMerit in the future.