YOU SAID:

but alas, he wrote her to come back fast because he was too swamped

INTO JAPANESE

しかし残念なことに、彼は忙しすぎてすぐに戻ってくるように彼女に手紙を書いた。

BACK INTO ENGLISH

But unfortunately, he was too busy and wrote to her to come back soon.

INTO JAPANESE

しかし残念なことに、彼は忙しすぎて、すぐに戻ってくるように彼女に手紙を書きました。

BACK INTO ENGLISH

But unfortunately, he was too busy and wrote to her to come back soon.