to have ended/to end

[Mr.Lucky_One]

"As we plan to have ended the work by next year September, we would be grateful for your early reply"
"As we plan to end the work untill next year September, we would be grateful for your early reply"

Which one sounds more naturally?

[5jj]

Neither sounds completely natural. I'd say, "As we plan to complete/have completed the work by September of next year, we ...".

[Gillnetter]

"As we plan to have ended the work by next year September, we would be grateful for your early reply".
"As we plan to end the work untill next year September, we would be grateful for your early reply".

Neither one is correct. In both cases you have put "next year" before "September'.

Which one sounds more naturally natural?

As we plan to end the work by September of next year we would be grateful for your early reply.

[Mr.Lucky_One]

What's wrong with "next year September"?
I guess I can still say "delivery terms" instead of "terms of delivery", so why can't I say "next year September"?
Is "next year September" incorrect at all, or it just sounds not very good?

[Gillnetter]

What's wrong with "next year September"?
I guess I can still say "delivery terms" instead of "terms of delivery", so why can't I say "next year September"?
Is "next year September" incorrect at all, or it just sounds not very good?

It doesn't make any sense in English. This is like saying, "Tire spare inner" instead of "Inner spare tire".