It is a specific term of hospitality and culture, generally asked by Arabs.
أي خدمة ؟ means Any help?
The detailed meaning is لو تامرني باي خدمة انا مستعد للعمل بها
"If you ask me for any help, I am ready to do it.
In view of grammar, it is in the place of جار و مجرور
جار or preposition is here understood.