Pay For You/Buy From You

[Bobbert]

I don’t think I was being understood when I tried to pay for some things for an older person. I would say my first statement, and then have to repeat it another way to make sure I was understood; but I don’t think I was saying it correctly, and I was probably just adding to the confusion when I tried to say it another way.

Are any of the sentences below correct and/or understandable? If not, what is the best way to say them using the same original verb?

I used the Tú familiarity because that’s the first thing that came out of my mouth, but I probably should have spoken to the person in Usted. So please show me the correct Tú as well as the Usted forms.

How close did I get? This is what I said:

I'll pay for your medicine for you.
Te pagaré la medicina por ti.

I'll pay for it for you (with an emphasis on “for you.” In other words, don’t open your purse and try to pay; I want to pay on your behalf.)
Te la pagaré por ti.

I'll buy your cell phone from you in a few days.
Compraré tu celular de ti en unos pocos días

Don't worry. I'll buy it from you.
No te procupes. Lo compraré de ti.

Input and explanations and appreciated.

[aleCcowaN]

I'm not sure about explanations, but here are what I consider to be the right versions.

(I suppose you're not offering to pay for someone's health insurance or debts, but for the drugs)

I'll pay for your medicine for you.
(Yo) Te pagaré los medicamentos.
or
(Yo) Pagaré por tus medicamentos.

Tus medicinas sounds Spanglish to me.

I'll pay for it for you (with an emphasis on “for you.” In other words, don’t open your purse and try to pay; I want to pay on your behalf.)
Te los pagaré
or
Los pagaré por ti.

I'll buy your cell phone from you in a few days.
Te compraré tu celular (o móvil) en unos pocos días

Don't worry. I'll buy it from you.
No te procupes. Yo te lo compraré.

[AngelicaDeAlquezar]

I agree with Alec. I will just add the Mexican nuance.

In Mexico "comprarle algo a alguien" may mean "to buy for this person" and "to buy from this person", but normally, the context will make the meaning clear.
If you want to say " I'll buy your cell phone from you in a few days."
I would say: "Yo te compro el celular en unos días."
And if you want to emphasize that you're not buying from anyone else, like in "don't worry, I'll buy it from you", you may say "No te preocupes, te lo voy a comprar a ti".

Now, for the medicines:
In Mexico, "medicina" and "medicamento" are synonymous, so if you're talking to a Mexican, you may use both words anytime.

-I'll pay for your medicine for you.
-> Alec's proposals are great. In Mexico, though, we don't use the future tense so often, so we may say:
- Yo te pago las medicinas/los medicamentos.
- Yo pago tus medicinas/medicamentos.
For reasons of style, we prefer to avoid the combination "te" + "tus"/"por ti"; although it may be used as an emphasis, in general it feels pleonastic, so it's not an elegant construction.
We might also have said: "Yo voy a pagar..." although this is better when you're telling the person in advance that you're going to pay for their stuff, or when you're emphasizing your will to do it.


Finally: When choosing between "tú" and "usted", considering age or status is not always useful. But be confident to say "usted" whenever you'd address the other person by a title (Miss, Mister, Madam, Doctor, Sir, Lady, Professor, General, etc.). And if you would call them by their first name, or by a familiar term (buddy, bro, mate, dude, and whatever the feminine equivalent words are ) use "tú".