Seller Offering new tenants before closing

[Natalie Olivo]

My partners and I are currently looking at our first deal in the hudson valley area. The place is a two family home (3 bed room, 2 bath) currently generating $1000 per door with existing tenants. The problem is that these rents are below market value according to comps in the area. The seller believes (or claims) the place could generate $1400-1600 per door and is promising to fill the place with tenants before closing. Does any one else have experience with this kind of deal ? How do we structure this so that we guarantee what the seller is offering?