So, this FREE glass?

yes

Why is it free?

because it was given to me. The reason it was given to me is because I
helped out the person who previously owned it. I offered them assistance
and advice with no expectation of any financial reward. They were in my
office and overheard a discussion about windows and made me what appears
to be a very good offer.

Scrap from some other glazier, seconds from a manufacturer?
My experience is that if something is free,
the donor is getting exactly what it is worth.
If it was first quality
goods, they would be selling it. Or, you are stealing it?

That last question says a lot doesn't it?