[Grammar] did not let

[Oceanlike]

I feel that this original sentence (b) is better than (a) but I am not sure. Seems like I should use "had finished" instead of simple past "finished".
Strictly speaking, are both acceptable? Why?

(a) He did not let the workers leave until they finished the work.

(b) He did not let the workers leave until they had finished the work.

 

[tedmc]

I think they are both acceptable, though the second gives more emphasis on the completion of the work.

 

[jutfrank]

Use (b). This is a case that is ideal for the past perfect, so use it.