Shinjuku Monogatari

Ibuki inherited an antique store, but to survive he takes side jobs as well. One day his frequent client Akemi offers him a job to fake kidnapping of a high school girl Megumi Naruyama, the only heir of Naruyama Real Estate group. Megumi herself is the initiator of the kidnapping. Since the pay is good - a million yen - Ibuki takes his younger partner Shinji and gets to the school gates.
But Megumi's uncle Sakai has own plans for Megumi and he sets a yakuza gang on the children trail.

[From vndbreview]