Are both sentences OK?
I will pay you back every last penny.
I will pay you back once I start earning money. / I will pay you back when I start earning money.And this;
I will pay off every penny.And this;
I will pay you back to the last penny.

Question

Are both sentences OK?
I will pay you back every last penny.
I will pay you back once I start earning money.  / I will pay you back when I start earning money.And this;
I will pay off every penny.And this;
I will pay you back to the last penny.

Answer

All of these are correct and common.
Good job!