grammaticality - "A" or "an" in front of a number that can be read in two ways

Consider the following sentence.

We are selling ___ 1,800 sq ft office.

I would write an, because I say eighteen hundred.
But someone else would say a, because they say one thousand eight hundred.

For 2,000 it's easier, because it's always two thousand.

For 2,100 it can again be twenty one hundred, or two thousand one hundred.

So what is grammatically correct, what do I write so that I don't confuse my readers?