If I intended to lie, that was

[Maybo]

I realized there was one thing I needed to do before Dad came, and certainly before the police arrived. If I intended to lie, that was, and have the lie stand up.

Source: Fairy Tale by Stephen King

What does "that was" mean?

 

[emsr2d2]

It clarifies the fact that whatever this "one thing" is, it had to be done specifically because of the plan to lie.
We don't use the phrase in the past tense very often although it works perfectly here. We use "that is" a lot.

Always write "capitalise" (when you're using British English, that is).
I need to buy a new dress - for the wedding, that is, not just for the hell of it.
I'll be watching the World Cup final on Sunday - the women's final, that is.

It can be used for clarification or further explanation.