"Negotiations are give and take, so we've debated that with employers on Friday. There has been development on that front, which we're not revealing until we've presented to our members," Jim told the radio station.
The union will hold a press briefing at 14:00 GMT.
The industry's main employer body, the Steel and Engineering Industries Federation of South Africa (Seifsa), last week "reluctantly" agreed to raise salaries by between 7% and 10% following a government proposal to break the strike that began at the start of July.
Union sources told Reuters on Friday that Numsa's regional branches had accepted the wage offer, but rejected conditions related to future wage negotiations.
Seifsa's offer includes tightening a clause known as Section 37, which is meant to prevent unions "double-dipping", or pressing for new demands at a factory or company level in spite of an existing industry-wide wage deal.
Numsa, a 200 000-strong union, argues employers want to make it "weak and toothless".
A smaller union, the United Association of South Africa (Uasa), with about 20 000 members, has accepted the wage increase "in principle".
The work stoppage has halted production at automakers and affected construction at new power plants in a country struggling with electricity supply.