Likely the offer isn't transferable, so you'd have to convince both buyer and seller, meaning you'd need to pay both of them off.
Contract wording is specific as to the buyer, and unless it has an assignment clause, it's not necessarily the case you can even do this.
The best thing to do, if this seller was using an agent, is approach the agent with your idea. Put an incentive in there for them, and ask that they approach the seller.
Lay it out cleanly, and easily. Make sure that the original buyer's agent still gets their cut. Keep it simple, if the numbers still work.
It likely won't happen, but that's the most likely way to try to make it work.
Good luck, let us know how it turns out!