Up and until the sale of the home actually closes, or ptoperty is lost in a trustee sale, the current owner may rent it out if they please.
The question is does the person renting the house know it is for sale and under contract? Are they aware that they will have to move once the sale closes?
Also, you will want your purchase agreement to stipulate that the seller is to deliver the tenant occupied property to you VACANT. I home should be VACANT before you do your final walk through.
Best of Luck to you!
Kawain Payne, Realtor