The gemara tries to show that devorim shebelev are not devorim in support of Rava from the case of a man who marries a woman with the understanding she is of the priestly class and it turns out she is not, the kiddushin is valid. But let us say devorim shebelev are devorim and the kiddushin should be invalidated because in his heart he thought she was a cohanes! This shows that devarim shebelev is not devorim. The gemara rejects the proof, stating that perhaps the reason for the kiddishin's validity is lehumrah. Rashe explains that it could be that devorim shebelev whether they are devorim or not maybe a safek, unlike Rava who maintains they are definitely not devorim. And the kiddushin in the case of the cohanes is valid only lehumrah. If we follow the logic of the gemara, it means that we have no proof that devorim shebelev is not devorim: it may be only a safek. Now if it is safek then what would be the din in Rava's case of the the man who sold all his property with the intention of going to the Land of Israel, safek. The din of safek is always that the mahazik remains with the item. In this case it is the purchaser who is the mahazik. If he remains with the property, then that's exactly what Rava says!! So what is the argument of the gemara to refute Rava??