You could gave him 50 merits that wouldn't be wrong either. But, the point is, your sending of merits shouldn't impact your and his relationship in anyway. Like if you send him 50 merits at a time, and to be honest he deserves those merits, then you may still not feel any hesitation when you may get to work with him in future.

We all are like a family on this forum, and if someone who appreciates a family member's posts or in simple words contribution then he/she can send merits to that member for his/her contribution. After sometime when the member who sent merits to a member gets accepted in a campaign that's managed by the same member then there's nothing wrong in that either.