Merit and grace are not equivalent…
We can not achieve salvation except through Grace. It is a ‘gift’ by definition. The real difference would seem to be tied up with the term ‘deserve’. To illustrate:
If I robbed a family blind of $10,000 and was caught. Now, this family taking my circumstances into account decide to not press charges - indeed they have said that I can keep the money, as I must need it greatly. I am let go, even though I have not paid the penalty for my crime - this is like ‘grace’
Now, if after that same instant I felt I was indebt to this family, I went out and cleaned their cars and mowed the grass, etc… The family however, feels that they must repay me for the service rendered - this is ‘merit’.
Note that without grace, I could never have received the ‘merit’ - they are tied together. Grace allowed me to ‘be ahead’ at a time when I clearly was not - but the family, being so awesome in their ability to aid, allowed me to be.