These transactions raise suspicion, the buyers knew it was a deal transaction, he should have increase the fee higher so that you can get it faster but decided to use lower fee, it looks like a flash transaction.

Your transaction doesn't have a replace by fee enabled, that means it will be impossible to bump the fee or for a child to pay for parent, by this you cannot do anything rather than to wait. Even the sender cannot do anything at this point, if the fee comes lower, it will confirm but I doubt if that will happen as today is just Monday(busy day), maybe you should wait if the mempool will be lower by the weekend.

If the transaction refuse to confirm and dropped, then you lose the domain and the money except if you are closer to the seller, you can tell him to resend.