But generally speaking, if I send you 10 merit and tell you "I sent 10 merit to you and now I don't have any smerit. Can you please send me 5 merit?", it would be still merit exchange.
Merit exchange is not rarely seen on the forum.
Assumption:
You can apply to be a merit source and get attraction of other users who will naturally or shadily exchange merit with you. It's hard or easy to judge a merit exchange based on how quality that merited posts are and the frequency of backscratching between two users or between a merit source and their local users.
If the merited posts are not too good quality but still receiving merits, and the frequency is high, that is merit exchange.
If the merited posts are good in quality, it is good to not consider it as merit exchange.