Problem with Neighbor over shed on my property

[Franky Juwana]

I flipped a house in the beginning on 2020 and project got held up due to Covid-19, so construction continued and it's done. 

After got an agreed offer, we went to contract and buyer's attorney found that my side neighbor shed is 1.5 feet over in our property, so they want to rectify by have them signing an affidavit saying that they can't claim the property. But my side neighbor saying that they have been there for a long time and would not move the shed, and if they have to, they want to be compensated by the new buyer. 

The buyer's attorney refused because it is illegal shed (because it's built on the side of their property and trespassing to my property). 

Now, I already holding the property for close to a year, and I want this property sold and move on to another project.  So I negotiated and willing to compensate $1500 for them to purchase a new shed, but today after, they said $4000 would be the price for them to sign the affidavit. And also mention about adverse possession claim after certain years. But they just want to be compensated and not claiming the property. 

I can't give them that much but not willing to drag this issue to court either which will take time and money, specially for me who already took more time that needed to do the flip. 

I really hope this will get resolved before it gets too expensive and time consuming.

Anybody had the same experience? recommendation? suggestion ? 

Thanks,