Q a) Which sentence is correct in the below examples?
Q b) If both sentences are correct then what are the meanings?
Context: I realised that our Payroll Processing Team has missed to pay salary of Jan 15 to one of our employee. I want to Email them that we need to pay him his salary.
We are to pay him salary for the month of January 2015 as we have missed to pay him.
We have to pay him salary for the month of January 2015 as we have missed to pay him.
Honestly, both of the options "are" vs "have" are ok, though "have" is better... "need" is actually a great option and I'd probably put it above "have" on a scale of which is more common.
The bigger issue, actually, is the "missed to pay him" part. This is completely ungrammatical and doesn't make sense... I mean, I understand what you're trying to say but it's very awkward.
Based on your added info, you have a couple of options:
We have to pay him [his] salary for the month of January 2015 as he did not get paid that month.
We need to pay him [his] salary for the month of January 2015 as we failed to send him a check for that month.
You can mix and match the first and second parts of the sentence as you like. You can also use "give" or instead of "pay" in either case or do something like:
We need/have to send him a check for the month of January 2015 because he didn't get his salary that month.
If someone is to do something, that person has been commanded to do it, or at least firmly instructed. It is not his choice. In the second person (you are to do such-and-such), it's a rather brusque way of giving instructions.
For your case, I recommend we need to pay... or we must pay... or we have to pay..., which express that your company has a need or obligation, but isn't being forced by someone else.
(And, of course, pay attention to idiom and grammar in the rest of the sentence: pay him his salary, since we still have not paid him for January.)
First off, when you fail to do something, you can use the verb miss in the patterns of miss + noun/pronoun and miss + -ing form, not to-infinitive. So you should say "......missed paying the salary for the month of ...."
You use the phrase "be to do something" to indicate a command, an arrangement made or a future action.
You use the phrase "have to do something" to indicate that it is necessary for you to do something.
Although the use of "have to" is more appropriate in the context of this sentence, the use of "are to" is also possible; it also makes sense.
To be + {infinitive} means to be expected to - in the sense that there is a a plan or process and the action you are describing is the next or expected step of that plan or process (or as @user8399 says, someone has told you to do it - making it a planned activity).
To have + {infinitive} can mean this, but more often means obligated to, compelled to, forced to, or required to in a more direct sense.
My car tire blew out, I have to get it replaced. (have is appropriate)
In a week I am to attend my sister's wedding (to be + {infinitive} is appropriate)
My car tire blew out, I am to have it replaced. (bad)
My wife's car tire blew out, I am to have it replaced tomorrow. (ok)
In your example, are to pay is appropriate since normally you pay salary on expected, planned times. But most people think of payment as an obligation when they perform work, so have to pay is OK as well.
The person doing the company's payroll would more likely use are to pay and the person who hasn't gotten paid is more likely to use have to pay.
