Confusing use of とたん in 「来たとたん何か用かよ」
While “Can I help you?” is one good way to translate 「何なにか用ようかよ？」 in some situations, this is not one of those situations.
Here, something like “So, you want a favor, eh?” would fit much better.
“As soon as I arrive(d), you want a favor, eh?”
I should perhaps mention the fact that “You want a favor (or something), eh?” is a far more literal TL of 「何か用かよ？」 than “Can I help you?” is. The latter is a fairly free translation.