Hence the if.
I said why it's possibly different than points.
And I said even if. I can't tell whether you were saying passing the offer through to the buyer might be a ribis problem, or keeping it might be a ribis problem, so I'll explain both.
If the cc holder passes the offer through, then the repayment from the buyer to the cc holder is $80, and therefore it doesn't matter whether the loan was $80 or $100 - there can't be ribis if the buyer repaid less than or equal to the loan.
If the cc holder keeps the offer, it's impossible to say that the loan from the cc holder to the buyer was less than $100. The sale was for $100, and the offer was never paid to the buyer - so how can the loan be $80? The only way to view it is as a loan of $100 from the cc holder to the buyer (which is repaid exactly), and a separate transaction between the cc holder and the cc company, which can't possibly create a ribis problem between the buyer and the cc holder.
Also, many here (Dan included) feel that points are a rebate on the purchase as well. So if you consider the amex offer to reduce the initial loan, you'd have to consider points to be in that boat as well.