How to word a specific subject/condition in a contract?

[Matt Blake]

Greetings, 

I'm writing up the subjects for a BRRR purchase. One major issue are the current tenants. They're awful, and the place is trashed, but price reflects that. I'm ok with it. The owner gave them notice, and a 'deal'. 2 month notice essentially. They pay MAY, get JUNE free, and once once they are out, they get a cheque for $1000 if moved before June 30th.

Our deal is set to close JULY 19th.

He obviously doesnt want to have subjects on there for 6 weeks, and the deal go south if on July 1st, the tenants decide to tell him to GFY when June 30th rolls around.  Dealing with some real winners, as you can imagine.  I say theres at least a 25% chance that this happens.  Regardless of the cash bonus, I doubt they have the capacity to make it happen.

I dont want this deal to go south, but to remove tenants is very difficult in my province, and he doesnt have a lease agreement with these tenants.  They've been there for 10 years.  

My thought is that if tenants have not gotten out by July 10th, that "$5000 cash back to buyer at close" would provide me with the cash to get rid of them.

How would I word this in the contract?