A couple quick questions here:
1 - Did you have a contract with the leasing company? Did you authorize them to negotiate & sign a lease on your behalf? (probably not, since you do not yet own the property)
2 - Did you put it in writing what you wanted in regards to a lease (rent amount, term limit, etc.)?
3 - The leasing company signed for the developer and not you? In essence, they (the developers) signed a new lease with the current tenant just before closing?
4 - Were you told that they were negotiating a new lease on the propertty when you looked at it, or did they tell you the current tenant's lease was up at end of Jan?
If you close on this property now, it appears (check local law, & subject to my understanding of the situation) that you would have to honor the tenant's current lease. That is, he will be able to stay in the unit for another 6 mos. at the rent level they signed him to. Once a lease is signed it is exceptionally difficult to break.
Additionally, where do the upgrades come in? Did they promise certain upgrades if you bought the uit? Was it in writing? Were there any contingencies (upgrades if unit is vacant at closing, etc.) in the promise of upgrades?
Sometimes a tenant will request a new lease if he knows his unit is being sold. They want the security that the new owner will not change the rent or other clauses on this as soon as they close. Read that lease carefully. Is the rent level acceptable? Are the conditions in the lease clauses acceptable? If so, go ahead and close and you can renegotiate a new lease with the tenant in the summer. Summer is an ideal time to find new tenants if he doesn't like what you want in the next lease. Good luck.