I am falling short by $100.

[tufguy]

John: Tom you owe me $200. Give it back to me.

Tom: "I am falling short by $100". I only have $100 at the moment. I will pay you rest of the amount later.

Tom: "I am short off $100". I will pay you rest of the amount later.

Tom: "I am short by $100". I will pay you rest of the amount later.

Tom: "I have $100 less at the moment". I will pay you rest of the amount later.


Please check my sentences. Are these sentences under comas correct?

 

[Tarheel]

John: You owe me $200. Pay me what you owe me.
Tom: I can only pay you $100 right now. Can I give you the rest later?

If you insist on using "short" Tom could say, "I'm short $100."

 

[PeterCW]

John: You owe me $200. Pay me what you owe me.
Tom: I can only pay you $100 right now. Can I give you the rest later?

If you insist on using "short" Tom could say, "I'm short $100."

In BrE we would say "I'm $100 short".

 

[Tarheel]

That would be equally natural in American English.