Fox had nothing to do with ROTJ they were only a distributor. Lucas put up the money himself, with a $35 million budget. Any decisions based on cost were his decisions, as Fox was not involved in the production itself.

You may be confusing this with the original film. Lucas had to constantly ask for more money. The film was initially budget for $7 or $9 million, or something like that, and I think it came in around $11 or $13 million. Lucas also asked for more money for reshoots, but didn't get quite the amount he asked for. I think Fox was pretty generous though, as it was one of the most expensive pictures released in 1977.