"No need for you to pay - I'll pay for this."

It is a friendly way to tell someone that you (The speaker) will pick up the tab. So if you are having lunch with a friend, and the bill arrives, and the friend says "put your wallet away" he is really saying "you don't need to pay, because I will pay the bill" Or, you don't need to take your wallet out, because I will be paying. The correct response is "Thank you" unless he has paid for you a few times already, in which case the correct response is "Thank you, but I am treating YOU today!"

other ideas: it's my treat it's on me foot the bill e.g. My boss took me out for lunch and the company footed the bill. You paid for dinner last time. Let me foot the bill for lunch today.

I'm not sure, but I think this expression means that someone has alredy paid for something(all) or only going to do it. I hope native speakers will correct me, if I'm not right :)

Like "stop bragging about your money", maybe?, it depends.