GM is reported to have asked EchoStar for $8bn (£5.5bn) in cash, in addition to its $30bn (£20.7bn) stock bid. EchoStar chairman Charles Ergen said he has a "high degree of flexibility" in structuring the deal, according to a report in USA Today.
However, GM has not allowed Ergen to begin formal due diligence proceedings and it has not given any indication when it is likely to start.
Ergen made a bid for EchoStar almost two weeks ago, just as it was looking like Rupert Murdoch was about to clinch a deal to merge Hughes with News Corporation's Sky Global Networks, parent of BSkyB.
GM is understood to want $5bn (£3.4bn) in cash from News Corp as part of its bid. The talks between the two are understood to be in the final stages.
Shares in EchoStar fell 0.79% to $27.72 (£19) on Nasdaq yesterday, while News Corp slid 1.9% to $35.12 (£24).