How bad does a sentence with an inicial 了 sound here?
他们请经理吃了饭- subtle emphasize what they eat, but not necessarily. 

他们请了经理吃饭- to emphasize who they buy dinner for

There's another possible way to say this: 

他们请经理吃饭了- subtle emphasis on the thing happened, but not necessarily.

It really depends on how you say it, your tune can change the emphasis too. They all have the same meaning. The second one probably has stronger effect. 

Please correct me if i'm wrong.