Can I deduct it from the tenant's deposit when doing the refund?

But this is in Dallas, Texas land of huge growth, so YMMV.I'd find a bank where you currently hold deposits.

I asked her to sign an agreement of early termination and paid all the remaining balance minus deposit by 3/31/2020 or I will reach out to collection company. 

The bank statements will likely help you more because they'll allow you to see when deposits are made. 

At least in my market, a lot of wholesalers are requiring significant nonrefundable deposits day 1, which is a risk I wouldn't personally take.

She paid first/last month rent and security deposit last month too. 

If they put an earnest money deposit down, contact the title company that was supposed to do the closing and see if they ever delivered the EMD and/or if it's still in escrow.

But she want us to get a mortgage denial letter to get the deposit back.

We were able to close the quickest, only ones willing to put 10% deposit with money going hard after DD.

If the seller agreed to do any repairs to the property you will want to view the property prior to closing to make certain the repairs were done (and done properly).Also make certain the current tenant security deposits are transferred to you.