The seller concession did not affect the sale price of the house. The seller agreed to accept 164k and pay for the septic problem. Aside from that.. it is done and signed.
The funds should be secured in title company account. No one can release them or cancel the check without your approval. These funds are part of the transaction and it is done.
But of course that is my opinion, you should speak to your attorney and tell him/her what you decide.
But, these are details of the negotiation and closing that we agents can only offer our general feedback and opinions, but negotiating and documenting points like this prior and through close is exactly why you have an attorney - talk to him/her pronto. Hopefully in the process the attorneys captured this agreement in writing.
Good luck to you.
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