Hi Elena, My first instinct was that the expression "Tell me if you need anything" could have the implied meaning of wanting to hep both in the present and future, depending on the context. However, if you want to make it clearer, you could say "Tell me if you ever need anything" or "If you ever need anything, tell me" You could also say, "Let me know if you ever need anything". Essentially, by adding "ever" it makes it clearer that you want to help someone both now and in the future. Hopefully that helps.

let me know if you need anything

It is clear from the sentence that you don't mean right now, but in the future. However, the future may be soon, e.g. in five minutes, or later today. If you want to explain that you cannot help today or tomorrow, but only at a specific time in the future, you could specify the time when you will help. For example: "Please tell me if you need anything after you arrive" "After you have received a reply, please tell me if you need anything"

It would be more normal in England to say. "Please let me know if you need anything" for now but also for close future "If you do need anything in the future please let me know". Notice we say please although we are helping you. If an English person takes out money at an ATM they will often say thank you. If someone walks in to me, I will say sorry.

I’m going to the store later. Tell me if you need anything.