I personally think he would only play for Newells if at all he would play for any other club - and regarding the money and all; Messi is well set - he co-owns Sirin Labs, a company that produces blockchain enabled smartphones; has his own chain of watches - Jacob & Messi; has a life long deal with Adidas; Could have numerous other investments that he has made too.
Don Andres playing in Japan - the primary push for him was to market his chain of wine in Japan; Xavi playing in Qatar - again, I doubt he's earned himself as much as Leo for example, so there was more push there.
Yes - money has the pull to get humans to do anything & everything - but, the family man that Leo is, with his study flow of income set for life, I doubt he would leave Barcelona! In fact, after all the heartbreak scenarios he has been through playing for Argentina and still getting blamed for their shit, I doubt he cares anymore to play for Newells either!!