Is there a time limit for revising or removing negative feedback after leaving it?

After exhausting all avenues with no seller response (major quality issue with an item) I left a negative feedback. Almost immediately (go figure) I got a reply offering a replacement which was what I had requested in the first place. They say they've posted it but it's not arrived yet and I'm getting 2-3 messages a day asking to revise feedback before time runs out. Today I get a message asking if I would accept a full refund plus keep the additional item and the original item on the proviso that I retract the feedback as soon as they do that.

The idea that they are that desperate for + feedback seems odd but it doesn't seem right that they want to refund plus replace for this to happen. Bear in mind I've not asked for anything more than the item to be the appropriate quality hence asking for a replacement product in the first place. They requested the feedback revision.