I don't think that it would be structured that way. As you say, they're not hard up for cash - but letting them have Sainz would be a show of good faith after their falling-out last year. Renault would get a quality driver and Sainz would get an opportunity to advance his career because the only way he will get a Red Bull seat is if somebody poaches Ricciardo or Verstappen. The only catch is that Renault would have to prove that their 2017 car is at least as competitive as the 2016 Toro Rosso, or else it's a step backwards for Sainz and Red Bull offering him to Renault is not a show of good faith, but turning him into a sacrificial lamb.