Is it permissible to sell open-source programming code, knowing that the seller is aware of the possibility that the buyer might misuse it to spread what displeases Allah, especially since the seller will not be able to identify the application after it has been modified?

If the applications are intended for permissible purposes, such as selling codes and publishing articles, then there is no harm in selling them, unless the seller knows that the buyer will use them for prohibited purposes. As for mere probability, it does not prohibit the sale. If the buyer purchases the application and the seller does not know of his intention to use it for forbidden acts, and then he uses it for such, there is no sin upon the seller.