Notwithstanding Mr. Von der ahe's assertion that he had no more players for sale, he has all but made arrangements for the transfer of Foutz to Brooklyn, receiving $5000 in return. The transfer of Foutz confirms the opinion that Mr. Von der Ahe is financially interested in the Brooklyn team. The negotiations for Foutz have been of recent date, and were probably commenced as soon as Byrne saw that Caruthers was likely to slip from his grasp. This makes the third player released to Brooklyn, but Foutz's loss will be little felt. He is no good as a pitcher and is a good fielder. His only redeeming feature is his batting. When questioned in regard to the matter, Mr. Von der Ahe stated that he would receive no players in exchange for Foutz. The consideration was purely a monetary one. Should the lean pitcher recover the use of his arm he may be a valuable man next year, but his arm has commenced to draw up like Charley Sweeney's did, and it is probably lost.-St. Louis Globe-Democrat, November 30, 1887
The sale of Foutz shouldn't have come as a surprise. It was reported in the Globe on November 13 that he would probably be sold and it was reported on the November 20 that he was "anxious to get away" from the Browns. However, on November 26, Von der Ahe stated in an interview that "I have no more players for sale" and that he was going to retain Foutz for the Browns. So while Foutz was one of the players who wanted out and there were rumors that he was going to be sold, it appeared, after the Caruthers sale, that Foutz was going to remain a Brown.
So what happened? Given the timing, it does appear that Foutz was sold to Brooklyn after it appeared that they wouldn't be able to sign Caruthers.
Caruthers was sold to Brooklyn on November 24 and was expected to sign a contract the next day. But that didn't happen. He held out for either a higher salary or a larger piece of the sale price. Brooklyn finally gave in to his demands on November 28 and thought they had a deal. At that point, even after all his demands had been met, Caruthers stated that he couldn't sign without the consent of his family, who promptly told him not to sign. He also mentioned that if he was going to sign at all, it would probably be in Cincinnati. Brooklyn was getting the run around and was no closer to signing Caruthers than it had been at the beginning of the process.
I find it completely believable that, at that point, Brooklyn went back to Von der Ahe and told him that they couldn't sign Caruthers and wanted their other pitcher/outfielder. The sale happened quickly because they had already talked about the possibility of Foutz a couple of weeks earlier. The groundwork for the deal had already been laid and they were able to get it done in twenty-four hours.
This seems to me the most logical explanation for why Foutz was sold to Brooklyn.