買ってやる

buy something for someone

→ 買う
phrase Uncommon #9,261
buy something for someone やる adds the sense of doing something for another person. It can sound casual or condescending, so it is commonly used for someone close to or junior to the speaker.
JA: トムに何か買ってやるよ。
EN: I'll buy Tom something.

私は彼女に新しいドレスを買ってやる余裕がない。

I can't afford to buy her a new dress.

買ってやる

I'll buy it for you.

— フォーリング・ダウン (1993)

服なら買ってやる

If it’s clothes, I’ll buy them for you.

— マルコムX (1992)

私は娘にカメラを買ってやるつもりです。

I’m planning to buy my daughter a camera.

そろそろ娘に適当なパソコンを買ってやる時期だと私は考えている。

I think it’s about time I bought my daughter a suitable computer.

これで携帯電話を買ってやる

I'll buy you a cell phone with this.

— グエムル -漢江の怪物- (2006)

毛皮でも宝石でも買ってやるからよな?

I'll buy you furs or jewels, so, okay?

— Tampopo (1985)

私は息子に腕時計を買ってやるつもりです。

I’m planning to buy my son a wristwatch.

買ってやるよ

I'll buy it for you.

— 自転車泥棒 (1948)