Something doesn't sound right. You need to meet with your Realtor ASAP or their Broker.
There was a date that the buyer needed to tell you that they would either accept or reject your response and if they were going to continue on with the sale or cancel the contract. If they asked you two weeks ago and you said no and I'm hoping all of this was in writing, then the ball was back in the buyer's court and had only a few days to decide if they wanted to get out of the deal. It also says that if they don't reply to you, they agree to accept the home as is. Not exactly in those words but that's the gist of it.
If they named a mortgage company in the contract, then that is the company they need to use and if they want to change it, an Addendum needs to be made and everyone needs to agree to it. If they couldn't get the mortgage with the original mortgage company by the date given on the contract, you can get out of the deal.
You should be meeting with your Realtor. Time is of the essence in an Agreement of Sale and it sounds to me like the dates are not being adhered to.
Also, it is your right to speak with an attorney and have them review all of your documents if you feel like you need legal advice.
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