I don't think Ola promised Fredo anything specific, and I don't think Fredo even inferred anything specific.

I think Fredo was being mostly honest with Michael about how the meeting with Ola went.

Ola recognized Fredo's desire for respect and just implied that Fredo wouldn't be stuck running Mickey Mouse nightclubs if the deal went through. That was all it took to get Fredo to betray Michael.

Fredo wasn't exactly a deep thinker, so I doubt he really considered exactly what he would end up with or how that would happen.