but if an individual post is deleted, either by a moderator or by the user himself, then the merits should get burned as @hilariousandco has said,
I will not agree with this because moderators are human of course that could commit human error in the process of deleting individual posts. If this will happen then there will be some members rank that will be demoted after deleting some of their merited posts? So, I do not think that it should be consider. I could be demoted too to newbie if it will going to happen.
If a mod removes a post that is unjust and didn't actually break any rules then you can complain about it in Meta (as I'm sure you would) and the post can be restored by an admin. If a mod removes a post then it is usually for a good reason or that it broke the rules and that merit probably doesn't deserve to be given in the first place.
because in that way, the one who sent the merits with the intention to abuse the system would get nothing back, neither the receiver would get any benefits out of that.
Merit abuse is something that should be stop but not in a way through deletion of the merited posts which considered as merit abuse.
If the problem is all about stopping the merit abuse then I suggest to consider a system that will approve and dissaproved merit give aways. It could be a lot of work but this will ensure that merit will not be abused.
Or else, there should be a control on how to give merits.
1-3 merits, for constructive and helpful posts.
4-6 merits, for sharing technical expertise. As we all know that technical aspects are being paid with high salary so we should consider also giving more merits on this posts .
7-10 merits, if it is helpful, constructive, encouraging, empowering posts, and sharing technical expertise.
11-50 merits - if the post was really brilliant and one of a kind post.
We should not worry about this because we could report the post/thread if found that there is a merit abuse being done. Like there are lot of posts that receive 50 merits that are relatively not that good and that is clearly merit abuse.
This is all completely subjective, and I'm not sure how you'd enforce it anyway.