I would not do this. I would definitely not do this by relying on a realtor’s word. This is way outside a realtor’s area of expertise.
First, I’m going to preface that my experience is solely in California, and the laws and terms may different where you are. You’ll need a lot line adjustment. To get that, you’ll need a new map prepared by a licensed land surveyor. After you have the map, you’ll need to take it to whatever government agency handles this sort of thing to get it approved and to change their official maps. The licensed land surveyor can probably advise you and your neighbor where to go, who to talk to, what it might cost, and how long it might typically take. How you pay for that is up to you and the buyer, but as others have noted, since you wouldn’t be doing any of this if the neighbor had not approached you, it only seems fair that the neighbor foot the entire bill, including whatever the licensed land surveyor charges for advice and guidance provided, inspection and permit fees, and any other associated costs.
Now for a cautionary tale. In the late 1960s, my grandparents owned a double lot in Palo Alto, CA. On one lot, there was their tiny house, a 2 bedroom, 1 bath place that was probably 900-1000sf, tops. The other lot was a pretty large vegetable garden and an unpermitted dwelling unit (it was originally a storage outbuilding that had gotten enclosed and turned into a 2-room dwelling we called “the shed”). Their next door neighbor on the side adjacent to the garden asked if they could buy about 15 feet off the garden property so they could turn their 1-car carport into a 2-car garage, and they needed a little more room to do it. My grandparents didn’t have a problem with selling them 15 feet, so my dad and uncle, both licensed civil engineers (their licenses permitted them to do surveys, too) drew up maps and took care of the details for my grandparents and their neighbors. The maps were approved, my grandparents were paid, the neighbors built a garage, and everybody was happy. Fast forward about 25 years, and this neighborhood had gone through a tremendous transformation. It was originally nothing but small homes of 1000-1500sf on relatively small lots, probably averaging about 6000sf. But by the mid-1990s, demand for houses soared, values skyrocketed, and tech immigrants were buying two adjacent lots to knock down the existing homes and build McMansions. A single lot commanded prices close to seven figures. A double lot was even more.
When my grandmother finally moved, she sold her property for something like $1.2M. Had it been a true double lot, though, she might have gotten over $2M. Selling 15 feet cost her a ton of money.
Unless you really need the money, I would not do it.