What is the meaning of 庸 in 利之而不庸? A random passage from Mencius is:   


  殺之而不怨，利之而不庸，民日遷善而不知為之者


The first two lines have the same structure V1 之而不 V2, wherein V1 is expressing someone doing something to another and V2 how this one reacts to it. 
I think I understand the first line 殺之而不怨: someone kills (some people from a group) and they (the group) don't begrudge it.

As for 利之而不庸 I only understand so far that: someone makes the group benefit from something yet the group 庸, of which I don't know the meaning. The meanings I found in different online dictionaries do not make any sense to me in this context. 庸 is translated as "common, unspecial, mean, low".
庸 means 酬谢 (reward) here.


  (10) 酬其功劳 [reward]
  (11) 又如:庸勋(酬劳有功的人)


And,


  6.酬其功劳。
  《书·益稷》：“明庶以功，车服以庸。”
  　孔颖达疏：“赐车服以旌别其人功能事用。”
  《左传·僖公二十四年》：“庸勳、亲亲、暱近、尊贤，德之大者也。”
  《孟子·尽心上》：“杀之而不怨，利之而不庸，民日迁善而不知为之者。”  


So 利之而不庸 means benefit them (the civilians), and they won't reward (someone).