Requirement for the payment of oath certificate in a job offer

I got a job offer from the UAE.

Now for the visa application form, they had to send me a form from the Abu Dhabi legal chambers as per my agency's request for an "affidavit of oath certificate" and they are asking me for AED 3300 for the certificate.

Is the oath certificate needed for a job in the UAE if the person hiring me told me that he'll pay it back when I reach Dubai?

Is this true and is there a so-called certificate?