Here's my take on your question.
1. Bibili ako ng mas maraming gatas bukas.
I will buy more milk tomorrow.
2. Ibibili kita ng bagong laruan bukas.
I will buy you a new toy tomorrow.
I think to easily differentiate this, adding "i" before the verb will make the verb's purpose for the object and not the subject.
Bibili ako - I will buy (for me)
Ibibili kita - I will buy (for you)
Also, it's more common to say "ibibilhan" when using this word. Changing the suffix of the root verb definitely gives some changes. But honestly... I think the example given sounds unnatural.
Here are some examples I can give instead:
Let's say you want to say [I'll bring you to my palace.]
Dadala kita sa aking palasyo. (doesn't make sense)
Dadalhin kita sa aking palasyo. (I'll bring you to my palace. Notice the -hin suffix after the verb. Adding -in or -hin to the verb will affect what will happen to the object)
Idadalhan kita ng palasyo. (I'll bring you a palace. Notice that not only the letter "i" was added as a prefix but "han" was also added as a suffix.)
Idadalhan kita ng aking palasyo. (My palace will bring you [something]. Tip: don't add a pronoun after the object. The meaning will definitely change.)