This is ambiguous to native speakers and, indeed, the source of endless interpersonal drama among English speakers.
"Do you have the money for the milk I got you on Monday?"
"Wait, I thought you said you were buying that for me?"
"Whoa, whoa, whoa! I said I'd buy it, since I was stopping by the store anyways, but I didn't mean I would pay for it!"
(Not a fictional example, alas.)
Also, allow me to add that I have as of yet been unable to find a way of disambiguating that expression that's sufficiently diplomatic as to not cause one party or the other to lose face.
This is perhaps why there's an idiomatic expression "I'll cover that for you" which unambiguously means that I will fund the purchase of something for you. Also "I'll cover you."