Can a seller be excused from a condition if the buyer caused the failure themselves?

Generally, yes, in principle — a party usually can't rely on a condition's failure to escape its own closing obligation if that party's own conduct is what caused the condition to fail in the first place, since that would let a party manufacture its own excuse to walk away. Many purchase agreements address this directly with language requiring each party to use reasonable efforts to satisfy conditions and not to take actions that would prevent them from being met.

Whether this actually plays out in the seller's favour turns on the specific facts — proving that the buyer's conduct genuinely caused the failure, as opposed to the condition failing for unrelated reasons that happened to coincide with something the buyer did, requires real evidence, not just suspicion. It also matters whether the condition existed for the seller's benefit at all; a seller generally has less standing to complain about a condition that existed purely to protect the buyer, even if the buyer's own conduct affected it.

If you believe the other side deliberately undermined a condition to get out of a deal, document the timeline and conduct carefully and get it assessed by a Treadstone business lawyer before assuming the deal is simply over.