Accidentally claimed to have a higher language certificate than i do... got offer and must present proofs.

I realise this sounds implausible, not to say plainly idiotic, but i got a French language certificate like 10 years ago, and for some reason it was always engraved in my memory to be a B2, so I casually wrote it in the resume, and worse... mentioned in the interview (although i claimed to remember nothing anyway). Turns out, it was a B1.... and now i need proof documents of everything to get hired. I would not like to get hired for a qualification i dont have. For my conscience and respect to the employer i have already sent an email notifying them of the mistake. But since the response will be in 6-7 days or so as usual, i would appreciate your thoughts on this and what to expect.

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