Manchester City's Serbia international was sent on loan to the Bundesliga club in January in a deal that expires at the end of the season.
But Schalke – who are fifth in the league – would like to see Nastasic extend his stay.
"If there is an option to buy on Matija Nastasic, that comes into force after a certain amount of minutes he played and Nastasic's first weeks in general," Heldt said.
"I always confirmed, and I do so now, is that we have an option to buy, but this option is conditioned on several situations which must arise. That is a good paraphrase.
"I can confirm that it is connected to situations which must arise. I don't want to narrow it down that it is just connected on appearances, but they are a subject."
Schalke's rise in the league came after a very slow start to the season, one that led to Roberto Di Matteo replacing Jens Keller at the helm.
Heldt lauded Di Matteo for the changes he has made, saying the Italian has just worked to Schalke's strengths.
"We conceded too many goals too easy in the past. And you cannot always score more goals than you conceded," he said.
"And that is why the approach was to stabilise the defence, to strengthen the organisation.
"These are two main topics we worked intensely on and which are crowned with success meanwhile, because we conceded less goals in comparison to the matches before.
"Our offensive potential has definitively suffered. But it is also explainable why it has suffered.
"The main topic is, that we have to battle with a lot of long-time injuries up front, which could make the difference, which could be creative, which could also make surprising moments.
"When Julian Draxler is out for several weeks, this is a problem for us. When Jefferson Farfan cannot play from start of the season on, this is a problem for us. When Leon Goretzka is out from start of the season on, this is a problem."
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