A man wants to collect money for poor kids. He met another man. The man wants to say that he already donated. How you say it in English? "No thanks, I already donated"? "I have contributed"? Or there is a phrase for this.
There's no set phrase; a polite sorry, I've already contributed or sorry, I donated already should be sufficient.
An American cliché applied in this situation is I gave at the office, shorthand for saying I have already given this cause as much support as I intend to give, so do not bother me again. Nowadays, however, it sounds abrupt, and I wouldn't use it unless you want to express annoyance at the request.
As an EL&U answer notes, it was a common-enough phrase to serve as the title of a 1973 episode of the sitcom Mary Tyler Moore. But while the United Way (successor to the Community Chest) is still a prominent organization, contribution habits (both in making and soliciting donations) have changed, and people under 40 may not be as familiar with it— it has an entry in Endangered Phrases: Intriguing Idioms Dangerously Close to Extinction by Steven D. Price (Skyhorse, 2011)
Campaigns for civic and charitable causes like the Red Cross and Community Chest were once far more prevalent at places of business than they now are, and people routinely made donations. Someone who was approached at home or elsewhere could have a valid excuse of "I gave through the office." By extension, the phrase came to be used to slough off any kind of request.